One option would be to reward the players if they have multiple sources for the same language, considering them an expert in some way.
Possibilities include:
* Bonuses to Charisma checks that would involve large words or complicated jargon
* Bonuses to teaching the language - Instead of it taking 250 days for a fellow adventurer to learn the language from you, it merely takes 125 days.
* Bonuses to Intelligence checks related to the culture of that language, that the character may have come across in their studies.