There is a thread about this:
viewtopic.php?f=14, but the OP stipulates all sorts of guidelines which I can't follow, so I'll work it out here first, and transfer conclusions, if any, over there.
My language study has never been orderly and systematic enough to compare in a quantitative manner -- some languages I've learned immersed, some I have learned without ever being interested in speaking them at all. I do, however, have a subjective view of which things have become easier for me. That might not be useful to the OP, but maybe somebody else will find it so.
I took to my L2 (English) like a fish to water in my first year of secondary school. Everybody was "wow, you must have a talent for languages", I was that good at it (now I think the talent argument is mostly nonsense). And I fell in love with languages. But then I didn't find my L3 (French) easier to learn than my L2. At all. The problem with using it as a proper counter-example to some spill-over effect from one's L2 is that 1. I started them both in secondary school, and I wasn't at a solid B2 in my L2 yet. 2. Also my L3 teacher sucked like a black hole, and almost single-handedly put me off languages altogether. 3. I didn't know much about self-directed study yet; that sort of thing wasn't encouraged, so I was at the mercy of whatever teaching methods were imposed by the various teachers. I struggled with my L3 in school -- not grade-wise; I did fine, but I never felt the way about it that I felt about my L2; I didn't connect with it. For a while there I thought "so much for talent", and then I took my L4 to test that theory (it restored my confidence). Years later I struggled with L3 again in its country of origin. Only now, decades later, when I restarted my L3 once more because I am stubborn that way and didn't want an abysmal failure on my record, am I progressing solidly, as I would expect from all the learning I have done in between. It is much easier now, so I am laying the early failure at the feet of my teacher, who sucked away whatever positive effect the L2 might have had otherwise. Cognates alone are huge, and we made almost no use of them.
My L4 (Russian) was the first completely unrelated language I learned, but I only got to A1 in it (again, high school, I ran out of time and then life interfered). I didn't feel any positive spill-over from my L2, which isn't to say there wasn't any, but I just was not aware of it and now I don't remember much about that time anymore. Learning my first foreign writing system was fun though. I had the best time just reading Russian out loud after I had learned Cyrillic, without understanding anything. That was also when I secretly smoked Gitanes. Ah, youth.
My L5 (Romanian) is related both to my L3 and L4, more to L3 with influences from L4. I was exposed to it on holiday, and then studied it back at home in between several more holidays during which I basically immersed myself. It took me several years (3? 4?) to get to B1. I never concentrated on it heavily except during the immersion periods of several weeks each. I loved the country; if it had not been under a repressive regime, I would have moved there. I slowly started developing my own ways of teaching myself, jettisoning much of what I learned in high school. I began to see the value of immersion; I learned so much faster when I was in-country. I started to experiment with pseudo-immersion.
I learned my L6 (Dutch) (closely related to L1 and L2) by full immersion while being actively engaged in acquiring it pretty much every day, quite rapidly, less than 2 years to C2 in all 4 basic skills; fastest I have ever learned a language (not counting Esperanto which will probably end up faster when I pick it up again). What I primarily learned was: relatedness is powerful, immersion is totally amazing, I can learn a language better without teachers than with even good teachers if I immerse myself and personalize my studies.
I learned my L7 (Swedish) fully immersed and engaged in less than a year to B2. It is related to my L1 and L6. This added further confirmation of what I had learned during learning my L6 -- those newly acquired study methods worked just as well for a less related language. After I convinced the Swedes that really, I wanted to learn Swedish no matter how well we got on in English, that is.
Skip over a bunch of A1 level language dabbling during which I stopped counting. Most of my later life had nothing much to do with languages, but I moved across the Atlantic and acquired native-like fluency in my L2. I have no idea how long that actually took. Probably because there wasn't actually a moment when I felt I got there, it snuck in under the radar. I've felt native for several decades now. More native than in my L1, which I speak so rarely that it actually takes some time now to activate it (speech only, reading is still just fine).
My current completely unrelated language is Japanese, in which my listening skills are at a solid A1, but other skills are limping behind; I have just started learning to read and write. I hadn't planned on Japanese at all, and consequently this was even more disorganized than ever; I just acquired it from listening to original anime and drama which one of my partners got me interested in. Any attempts at learning it in a more organized fashion before now failed mostly because my depression had become so debilitating that I've had to first new ways of learning because many of the old ways no longer make it happen.
Japanese in general is usually classified as one of the hardest languages for speakers with my background. And yet I can't say I am finding it particularly hard. Sure, the writing systems are different, but that doesn't feel "harder" per se, it just requires more memory work up front and much much more writing practice just for the writing itself, not as a memory aid. The grammar itself at this stage seems (maybe deceptively) simple. I do perceive some benefit from other languages in Japanese -- that I've encountered SOV before helps because that seems to require some mindset switching and I already know how to do that; that I've learned another non-Latin writing system helps, even though Cyrillic is child's play in comparison with kana and kanji.
I generally perceive little cost from interference at all. Unless the mere fact that there is more than one language I pay attention to counts as interference; I don't see it that way -- anything I engage with at all takes away time and energy from everything else. Unrelated languages do not interfere for me; related ones will interfere if I am learning the same things at the same time; but If I have a head start in one, I won't get confused. Confusion is mostly about vocabulary anyway, I don't seem to get confused about grammar. One area I've noticed as problematic: using the same alphabet is a hindrance when one language has pronunciation that differs a lot in only a few areas from another language I already know. So, French was no problem, but Gaeilge was -- I'd much prefer if it used a different alphabet. I've since been wondering whether it might have helped if I used Gaelic type during the initial stages, whether that would make it sufficiently different. When approaching a new language I used to always start with reading after a basic overview and practice of pronunciation, after that experience I shifted to drilling pronunciation seriously first. That works better even for languages where I don't have an initial pronunciation problem.
Direct benefits IMO definitely occur between related languages -- I am currently studying French and Spanish. Spanish massively benefits both from cognates (more vocabulary faster, which means that two months in I can already read simple native materials), and from similar grammar. I'm thinking of picking up Italian once my Spanish is at A2 because this effect is too good to not exploit.
Most benefits I see come from heaving learned another language to fluency, and having learned how to learn at different stages in the process. I was positively motivated by my early success with my L2, and even the failure of the L3 didn't do entirely away with that. I've tried a lot of different methods, I've figured out what works, what always fails because it's just not suited for me, what fails temporarily, like when I am stressed, what works at the very beginning, and what works better once one can read well enough. I am no longer thrown by small set-backs and by plateaus; I know how to take the former in stride, and break the latter.
I am Jar-ptitsa and my Hawaiian name is ʻā ʻaia. Please correct my mistakes in all the languages. Thank you very much.
