I suppose that the tagging UI is quite trivial, you just need an single icon that iterates the languages in loop when pressed. And many users will have just one language, so they do not have to touch anything after the onboarding.
Then using a service you cannot "upgrade" to the region labeling, and you have to manage the incorrect detection issue; finally a service have be manteined and be economically sostenibile. Better to offload this easy task to the users.