Learn 12 languages in 12 months? Right.

[Le Baron]

Even just native English → French transparency is huge. It's a standing joke in our household that you can just take any big English or French word, and change the pronunciation to get the other language.

This is only ever more advanced vocabulary transparency and limited by the problem of a huge number of false friends. English in daily use language is almost entirely a Germanic language. Nearly all its common words are of that origin.

And as s_allard pointed to there is the rather noteworthy grammatical (and syntactical) difference, plus structures that simply don't exist in English. The basic structure of French is nothing like English, there are just some similarities sometimes which is misleading. In fact this is perhaps the initial hurdle in both directions. A friend of mine whose first language is French (though one parent is Dutch) returned to NL after schooling in France and really struggled to learn Dutch. When they started with English he also struggled with that, and it was only when he reached advanced vocabulary that the overlap started. As such he uses those words more and tends to sound 'educated' in English, but stumbles more over what would be seen as 'simpler' English.

There's a lot of to-and-fro in this thread and I wouldn't say that I completely reject every last bit of what s_allard is putting forward, except for the time factors and limits on vocabulary. Since it is possible to be somewhat functional with a vocabulary of a limited size. Even though this will be full of holes and handicap you at times. Doing a lot with a little when core grammar is well understood has its place. I still subscribe to the approach of: a little grammar well understood and as many words as possible over time.

The main subject at the core of this thread is time. As in: what can you realistically build in x amount of time? I say that a month won't give you a great deal and even three months is only scratching the surface; even if a person believes they are paring down everything and planning for efficiency. It's not a realistic prospect. You'll have something, but it won't be much.
The questions around getting a leg-up from other langages are being suggested in a disingenuous manner. If I 'start' Spanish today, but actually I've done it before several times in the past and also know Italian, I am not really performing what the question implies. As leosmith remarked, try it with some difficult and perhaps unrelated languages and see this masterplan fail miserably. Additionally, other aberrant and anomalous exceptions keep being offered to make the peg fit the hole. That someone COULD perform these feats if they studied 10 hours a day. That they COULD learn vast numbers of words in small amounts of time if they employ this or that exaggerated stratagem.

[tastyonions]

Even just native English → French transparency is huge. It's a standing joke in our household that you can just take any big English or French word, and change the pronunciation to get the other language.

This is only ever more advanced vocabulary transparency

le point, le train, la personne, le cercle, l'océan, la question, la cour, le cours, la partie, la question, le moment, l'instant, l'effet, le bus, le groupe, le problème, le rapport, la vue, l'ordre, le service, la face, la force, la situation, le projet, l'idée, la région, le sens, le nombre, le compte, l'exemple, la mesure, l'histoire, la loi, le marché, la condition, le système, la société, l'entreprise, la manière, le prix, le terme, le courant, l'intérêt, l'information, le détail, la liberté, le programme, la ligne, la sorte, le sujet, l'action, la relation, le doute, le reste, le début, la présence, la forme, la décision, le rôle, le produit, la minute, le peuple, la seconde, la position, le développement, le chef, l'économie, l'effort, le membre, la beauté, le titre, l'objet, la période, la voix, la base, l'occasion, le cousin / la cousine, l'oncle, le choix

Most of these I took from the top 500 most common French words in the nouns section of my frequency dictionary. Not exactly advanced stuff.

[iguanamon]

...As the old-timers around here like emk, iversen and iguanamon know, I’ve always argued that a relatively small vocabulary -- not limited to the 300 most common words – combined with impeccable grammar, idioms, formulaic speech patterns, discourse-making strategies, writing practice, good phonology, reading one fat book, and exam-focussed preparation makes for great results. For me.

I was impressed with s_allard's relatively quick advancement in German. He backed it up by his result in the test. He's got every right to be proud of his accomplishment. He has nothing to prove.

People can argue about whether CEFR tests are indicative of true ability in a language. They're not perfect, but it's the best we've got, short of actually interacting with native-speakers and receiving their opinions; observing what you can accomplish with the language and the results. My CEFR tests are in the real world, interacting online and traveling (successfully) dealing with all kinds of problems and issues in TL countries from Brazil to Portugal and Puerto Rico to Argentina and Spain. I've passed.

I live in an isolated area in the Caribbean. Leaving island involves flying and is expensive. I can't just pop out for the afternoon and take a CEFR test and be back home for dinner. I'd rather spend that time on traveling. I can never justify such an unnecessary expense for myself.

As far as the original topic. I haven't followed the link. To me, it's a gimmick. We all know it. Can you learn a lot in a short amount of time- one month specifically? One month is arbitrary, but yeah, you can. Can you retain that knowledge as usable after the arbitrary one month expires? That's the question. It always comes down to defining what "learning" means.

As someone who has learned a few languages to a high level, I can say that it takes me time to learn and consolidate that learning into effectively using the language. I do not care how much time it takes me to do it. I care about results- being able to have meaningful conversations with people; being able to enjoy native media; being able to travel in TL countries without having to fall back on English- that's my CEFR test. It's not about how many hours I've spent. I don't count them because for me there's no point in doing that. It's not about how many words I know. I know enough to be able to do what I want to do in, and with, my languages, which is what matters to me.

My formula is: Learn the basics first; Read a lot; Read widely; Listen a lot; Listen widely; Speak when you can; Write. This takes quite a bit longer than a month- which is why I think 12 languages in 12 months is a gimmick. If it sounds too good to be true...

[Le Baron]

Even just native English → French transparency is huge. It's a standing joke in our household that you can just take any big English or French word, and change the pronunciation to get the other language.

This is only ever more advanced vocabulary transparency

le point, le train, la personne, le cercle, l'océan, la question, la cour, le cours, la partie, la question, le moment, l'instant, l'effet, le bus, le groupe, le problème, le rapport, la vue, l'ordre, le service, la face, la force, la situation, le projet, l'idée, la région, le sens, le nombre, le compte, l'exemple, la mesure, l'histoire, la loi, le marché, la condition, le système, la société, l'entreprise, la manière, le prix, le terme, le courant, l'intérêt, l'information, le détail, la liberté, le programme, la ligne, la sorte, le sujet, l'action, la relation, le doute, le reste, le début, la présence, la forme, la décision, le rôle, le produit, la minute, le peuple, la seconde, la position, le développement, le chef, l'économie, l'effort, le membre, la beauté, le titre, l'objet, la période, la voix, la base, l'occasion, le cousin / la cousine, l'oncle, le choix

Most of these I took from the top 500 most common French words in the nouns section of my frequency dictionary. Not exactly advanced stuff.

This is not quite the bulk of actual everyday common words indispensable to getting on with English. Let me take just a few I've randomly put together and look at the disconnect with common French. And how these map far more onto German, Dutch or a Scandinavian language. Excuse my scrappy formatting:

Be – être
And – et
Of – de
A – un, une
In – dans, dedans, à, en, chez
To – à, pour, de, par, vers
Have – avoir
Too – en plus, aussi, également; trop
It – le, la, ce, il, elle, ça... (long list)
I – je, moi,
That – Pronoun, conjunction or adjective, it's long list of words that have no relation to the English one.
For – Pour; but also prepositional 'depuis' or conjunctional parce que..
You – None of them are recognisable from English.
He – Or these.
With – Avec, à, de and a few other prepositions.
On – Sur, but also 'au' indicating 'at' (e.g. a time)
Do – The verb 'faire' as used is simply not easily mapped onto 'do'.
Say – Dire.
Hold - Good luck dealing with 'tenir'
This – Same issue as 'that'.
They – Various possibilities.
At – Au, à, de, sur, en...
But – Mais...but also versions 'however'
We – Nous/on
His – Son/sa, ses, le sien/la sienne
From – another long list of possibilities with no relation to 'from'
Not – Refer to grammar of French negatives!
Can/can’t – Completely different construction
Won’t – Same story
By – “hahaha
She – Same as he
As – Comme, aussi (que), en tant que, ainsi que, alors que, au moment que...
What – Quoi, que, quelle/quelle
Go – Aller, plus conj. 'vais, va' etc; passer, partir
Who – Qui
If – Si
Would – Utterly different construction built into each verb
All – Tout/toute/tous etc
My – Mon, ma, mes
Make – Faire (wait a minute wasn't that 'do'?)
About – à propos, environ, sur..., s'agir de
Know – laughably long list of possibilities
Up – en haut, au-dessus de..en l'air!
One – On, un/une,
Time – Heure; fois etc
There – Là
So – Si, tellement; donc, alors.
Think – Penser, plus a large number of idiomatic possibilities
When – Quand, lorsque, lors de,
Some – Quelques/certains, but also e.g. constructions using 'en'.
People – Gens, monde, peuple
Take – Prendre, emporter
Out – Dehors, sortir
Just – Juste, seulement; but also: 'je viens de..'
See – Voir
Now – Maintenant, actuellement
Than – Que/de
How – Comme
Its – None of them resemble this.
Our – Notre/nos
More – Plus, davantage de, encore
Want – Huge number of choices none of which resemble 'want'
Way – Same problem.
Because – Parce que, car; (as conj.) autant plus
Find – Nothing resembling 'find'
Thing – Chose
Well – Bien...

Not so 'transparent' after all.

[tastyonions]

If your point is that pronouns, conjunctions, and prepositions don't map well between French and English, well, duh, that's obvious. But there are quite a lot of words that do map well, many of them not very "advanced" at all.

[Le Baron]

If your point is that pronouns, conjunctions, and prepositions don't map well between French and English, well, duh, that's obvious. But there are quite a lot of words that do map well, many of them not very "advanced" at all.

But I provided only a handful of very basic and common and necessary words, whose basic translations are completely different. English > French is not 'transparent'. It's a superficial myth.

[tastyonions]

I see transparency as a matter of degree. English -> Basque is basically 0%. English -> French is some much higher percentage. No, it's not as simple as going from Swedish to Danish or Spanish to Italian, but it's also quite far from the English to Basque example.

[Le Baron]

I see transparency as a matter of degree. English -> Basque is basically 0%. English -> French is some much higher percentage. No, it's not as simple as going from Swedish to Danish or Spanish to Italian, but it's also quite far from the English to Basque example.

Well we could say English > Chinese/Japanese/Russian/Turkish is less transparent, but it doesn't thereby render French as transparent as is so often being claimed. It's obvious that languages with overlap will be. In the case of e.g. Dutch it's because of actual structural relation (even though actually loads of Dutch vocab is obscure to English). French's overlap is different and mostly due to 1066, not basic structural similarity.

[iguanamon]

I think to us as language-learners and native English-speakers, French is more transparent than it would be for a monolingual. We tend to forget that as language-learners we see patterns more often than monolinguals do. Of course, I have two French Creoles, Spanish and Portuguese under my belt. I can definitely understand a lot of written French thanks to my languages- native and learned. No way can I speak it... nor would I try.

If I ever decide to learn French, I don't think my learning wouldn't take nearly as long as it would if I were approaching it from scratch. I have discounts.

[Cainntear]

I forgive your bluntness but I will also point out that unlike most people around here I have actually sat three CEFR exams, two of which successfully : C2 level (Spanish) and B2 (German) both at first go. My B2 Italian was not a total sucess but I considered it more of a pleasant experiment. Now I’m thinking about having a go at C1 or C2 in June. So I think I know a thing or two about preparing for CEFR exams.

People can argue about whether CEFR tests are indicative of true ability in a language.

There is not really such a thing as a "CEFR test".

There are many tests that give you a grade against the CEFR. Some of these are from institutions that are members of the ALTE --the group that set up the CEFR -- but that doesn't really give them much of a claim to be the only "true" CEFR tests. If the exams are similar to each other, great, but that's a bit more to do with examination culture and a freer exchange of information between examining bodies, but there will be exams hosted by ALTE member bodies that are very different from each other.