They will use the same language code as everyone else. Which currently is, if the spoke language is in your language, or you have universal_speak or universal_understand, you can understand it.

It seems like before the responsibility for whether you can understand was awkwardly split between the language system and if-statement checks. Keeping it consistent simplifies things a lot, as it makes it clear that the language system is responsible for languages, and the if-statements are responsible for the rest.