It's interesting that, if really Tolkien was the D&D inspiration for inclusion of languages, they botched it so badly as to have racial language, when Tolkien specifically didn't create elvish, but quenya and sindarin, because the race was split long enough to have evolved two different languages.
Languages in D&D are weird because they have a chance of being chosen before the campaign pitch. There are some games where character choices inform the GM about what the player wants, but I don't think it's often enough the case here to assume that if a character elects to learn Sahuagin, then the GM should feel forced to include some Sahuagin... he might have a desert-based campaign instead.
The weirdness also comes from D&D having no default world, as was mentionned already above: in the example, they have to resort to race language because anything else wouldn't really fit (class-based is weird, even Thieves cant, and alignment-based is outlandish...)
Also, there is a mechanical cost to learn new languages, and not all campaign can afford the required downtime.
All those flaws, however, doesn't mean they should be discarded in favour of a weird pre-Babel world where gods introduce languages into sapients brain (if they can put a language in their creatures' mind, they could also put there a fear of X and it's a ship that has sailed). It should be divorced from character creation as it shouldn't be a mechanical part of the character but a part of its backstory.
How to deal with language should be moved to the DMG with advice on how to use them and say "Everyone is speaking English for some reason in this world" if one doesn't want to deal with it, and introducing different options based on campaign scope and probability of actually having to use those languages.
[Assumption that Common = English is also something fun... I have seen adventure where an NPC was thought to be a male and was in fact a female, and yet the PC could find a piece of writing from said NPC... that would give away the gender unless being very careful in other languages where adjectives or worse verb carry a gender mark].
Languages in D&D are weird because they have a chance of being chosen before the campaign pitch. There are some games where character choices inform the GM about what the player wants, but I don't think it's often enough the case here to assume that if a character elects to learn Sahuagin, then the GM should feel forced to include some Sahuagin... he might have a desert-based campaign instead.
The weirdness also comes from D&D having no default world, as was mentionned already above: in the example, they have to resort to race language because anything else wouldn't really fit (class-based is weird, even Thieves cant, and alignment-based is outlandish...)
Also, there is a mechanical cost to learn new languages, and not all campaign can afford the required downtime.
All those flaws, however, doesn't mean they should be discarded in favour of a weird pre-Babel world where gods introduce languages into sapients brain (if they can put a language in their creatures' mind, they could also put there a fear of X and it's a ship that has sailed). It should be divorced from character creation as it shouldn't be a mechanical part of the character but a part of its backstory.
How to deal with language should be moved to the DMG with advice on how to use them and say "Everyone is speaking English for some reason in this world" if one doesn't want to deal with it, and introducing different options based on campaign scope and probability of actually having to use those languages.
[Assumption that Common = English is also something fun... I have seen adventure where an NPC was thought to be a male and was in fact a female, and yet the PC could find a piece of writing from said NPC... that would give away the gender unless being very careful in other languages where adjectives or worse verb carry a gender mark].