I can make a list now and discover in few years that I changed my mind
but I try:before I get assaulted: all the languages are beautiful and no language is less than others.
- Japanese, it's true that I contemplated learning it because of a travel in Japan, but please, nope. It has all the characteristics of a language that I hate: language isolate, 3 alphabets, one of which is not even an alphabet but a set of ideograms, so weird and alien and strictly culture dependent that if you don't live in the country you basically have no chance whatsoever.
- native american languages. I'm not American and I will never meet a Navajo person or read Navajo books (do they even exist)? Plus they're difficult as fuck.
- languages that are extremly endangered and extremely isolated.
- Basque. How was the thing? The devil created Basque and then tried to get fluent in it, failing big times? Or was it Japanese?
- Most of the conlangs, especially the ones that have too less in common with natural languages, because they're not useful as a bridge. Esperanto and perhaps Slovio can be interesting for me (but I don't have to speak them or learn how to write them). Yes, I had some fun with Toki Pona.
- Judeo Español and Yiddish, unless I studied Hebrew first.
- Faroese, unless I studied Icelandic first.
- Urdu, unless I studied Hindi first.
- Thai and Vietnamese don't score particularly high in my language wishlist, but I reserve myself the right to change idea.
- Turkic languages written with the Arabic alphabet, although I can always learn all the others first and those only by listening.
- Dialects. A waste of time unless I live in the area where the dialect is spoken - and often it is a big waste of time anyway.
- Neapolitan dialect. If I have to decide which languages sounds more lovely between Klingon and Neapolitan, I would certainly vouch for Klingon.
- Frisian. Friesland doesn't exist: it's an invention of the communists. Unless they send me in Frisia to work
- Limburgish: a big who cares. Same for Luxembourgish.
- Swiss German: bleah.
- Romansh: a huge who cares.
- Ancient Egyptian and all the extinct languages that don't have any related modern language.
I'm sure there are many more to add, but this is a short representative list.