While reading this answer to whether or not a druid knows the languages of his/her chosen form, I began to wonder, "What is a language exactly?" Please allow me to walk you through my thought process. I hope these aren't considered as separate questions so much as things that help clarify my problem.
Is it a skill? I can't find anything in the PHB or DMG that relates skills and proficiencies to language. In fact, when both are mentioned in a given header, they are never mentioned together as the same thing. I can't seem to find out how you learn a language, though I'm sure rules exist that might shed light on the issue.
Is it part of the stat block? When creating monsters in the DMG, there is a table for statistics, which doesn't include ability scores. So I don't think language is part of the statistics, which would contest the idea that a druid only knows the languages his Wild Shape form knows (see the linked answer above). True Polymorph has similar language to Wild Shape (where this question stems from), and makes no mention of language other than to say you can't speak unless your new form is capable of speaking. Otherwise, language is either omitted or assumed to be part of creature statistics, ability scores, and/or skills.
Is it related to attribute scores?
I've heard that things with low intelligence don't know languages, but I can't find any proof to back it up. If that were true, perhaps language can be considered to be tied directly to an ability score and therefore not part of the stat block. This would also mean that modifying ability scores could potentially affect language speaking ability as a RAW effect, but might not make sense to change the languages you do know.
Is it part of personality? Feeblemind mentions "Shattering its intellect and personality" by reducing intelligence and charisma scores to 1. Personality is also mentioned in True Polymorph and Wild Shape, which might lend credence to the idea that language is tied to personality OR it might mean it's tied to ability scores. Feeblemind also specifically mentions that the target can't understand or speak any language, presumably as a result of intelligence and/or charisma being 1. If language were tied to personality, changing your form wouldn't affect language as long as you retained your personality.
Is it it's own thing?
Perhaps languages aren't related to anything and this might mean there is no hard and fast rule regarding what languages are and how they are related to the technical aspects of the game.
So what is a language? Perhaps to get behind this question is to ask "What are the implications of changing your ability scores, statistics, and skills for language?"
Best Answer
Languages don't come from stats, ability scores, or skills. They come from race, and possibly from class or background.
From their first mention languages are set out as a racial benefit. Two exceptions arise--Druidic and Thieves' Cant--as class benefits. Later, we see that knowledge of a language may arise from a background (p.125) or from extensive training (p.187). Which gets us to my interpretation:
Languages are part of your deep background.
Language acquisition as a racial feature arises from assumptions about segregated communities; the DMG (pp.20-21) discusses ways in which one might adjust these assumptions and how that might impact language acquisition. (They give the example of a racially-blind kingdom-dependent language system as an alternative.)
Language acquisition as a class or background feature is more-explicitly based on long times spent in the relevant community/diverse settings/life of study. Note the acolyte and sage gain two languages; guild artisan, hermit, noble, outlander each gain one. Alternately, you can pay to train for 250 days and 250 GP. This also constitutes a large amount of time, effort, and investment on the part of the character.
Thus, changing your ability scores, statistics, or skills have no effect on your languages. Because those changes haven't changed your experience, by which you acquired language. (Admittedly, those changes might impact your ability to hear, speak, read and/or write, however.)
Languages aren't a skill. They aren't tied to an ability modifier and your broficiency bonus in that way because they're binary: you can't (RAW) be more- or less-skilled in a language.
Languages aren't pegged to attributes. But they used to be. Originally PCs were guaranteed two languages: common and alignment. INT>10 made it possible to know more languages. In 1e your intelligence capped the number of languages beyond your base (racial/class) languages that you could know. In 2e your intelligence capped the total number of languages that you could know, all the way down to INT=1 capping you at zero languages: "while unable to speak a language, the character can still communicate by grunts and gestures." (PHB1e p.10, PHB2e p.16)
[ed.: I'm working off of retroclone material for the Original cite: if anyone's got a good cite to edit in I'd appreciate it.]
Is it related to personality? Personality isn't really a defined term in D&D, so this gets sticky, fast. For our intents I think it's easy enough to say "no," but recognize that language and personality formation are interwoven in real life in a way our game just isn't trying to simulate.