But also, for most purposes, you don't need to. Just click on the little gear icon in the "proficiencies and languages" box on the character sheet, and add the language from there. (I'm sure you can do it in the app, but I haven't checked how.)
(I'm assuming this is to give people a specific language, because Linguist already exists.)
How do I create a feat that grants the character knowledge of a specific language?
-> Modifier
-> Modifier Type - Language
select the language.
2014 5E mostly
3.5 maybe.
But also, for most purposes, you don't need to. Just click on the little gear icon in the "proficiencies and languages" box on the character sheet, and add the language from there. (I'm sure you can do it in the app, but I haven't checked how.)
(I'm assuming this is to give people a specific language, because Linguist already exists.)