From what I’ve seen, it depends on how much is being spoken, and very much depends on what the actor is capable of. For a single line, more often than not the actor will learn it phonetically, and just say it themselves. If it’s a whole episode, the actor may accept the challenge, but if confidence isn’t high, they’ll get a sound alike. For full translations, like anime dubs, they get all new actors. But sometimes the same actor will stay and do the other language if they are fluent enough in it to do the job well.
Then there’s Vin Diesel doing all the alternate language versions of his groot line himself, which a lot of people tout as amazing, but I feel like learning how to say one thing in a foreign language, and just saying it with different levels of intensity or cadence, is something that anyone can do, and shouldn’t really be celebrated.