This is a list of language-related Stack Exchange sites. If your target language is not listed here, please consider creating a proposal on Area 51.
The sites are listed in alphabetical order. As long as a proposed site is in the definition stage, questions can be submitted and upvoted but not be answered.
All such proposals are currently closed.
Update March 2021:
For a list of other language-learning resources not hosted by StackExchange, see this post.
Sites about languages in general, but not about a specific language:
Language-specific sites, in alphabetical order by language:
On Psychology & Neuroscience Stack Exchange, some questions tagged
language-processing may be relevant to language learning.