Arelith

Each familiar is fluent with a few languages, however, it appears that they're not fluent in speaking that language.

Below is a test using a Worg familiar that is fluent in goblin and animal language, with it displaying a clear pattern of failing language checks when speaking those languages when possessing the familiar.

My character is fluent in none of those languages, for instance.

This doesn't seem intended, as the languages work as intended when using the -a command to speak through the familiar.

the familiar can't speak those languages because you are possessing it; your character is quite literally speaking through the familiar, therefore it is limited to whatever languages the character themselves is fluent in

in theory, while possessing the familiar, it should also be able to speak any other languages that your character is fluent in, even if the familiar otherwise cannot

whether or not this is actually intentional however, i have no idea (though personally i would prefer it to be consistent)