Over on Area 51, one of the proposed names for this site was Second Language Acquisition. While the term does accurately describe the scope of the stack, the part that really bothers me is the "Second Language". Questions on this site are not only restricted to a second language. It's perfectly fine if you have any questions about learning a third or fourth language. For that reason, I suggest we change this name proposal to "New Language Acquisition" or something like that.
Just like "dual citizenship" is usually used to refer to two or more—not "two and only two"—citizenships, I understand "second language" to mean "not the first", rather than "between the first and the third". So I don't think your concern is warranted.
a language that is not the native language of the speaker, but that is used in the locale of that person. In contrast, a foreign language is a language that is learned in an area where that language is not generally spoken.
So one could make the argument that it's excluding foreign language learning, but not that it's excluding learning a third or fourth language. It's simply not what that term means.
That being said, the current title "Language Learning" seems very clear to me, and I don't see why it would need to be changed.
For future readers: questions about native language acquisition, including re-acquisition, are also on-topic. So the name "Second Language Acquisition" would be a bad fit for the site.