It varies - TVTropes has a long list of examples. One common convention when media is made in language A and uses language B, then gets translated into language B, is to translate language A to language B and language B to some other language. For example, the TVTropes page lists an anime example where an English teacher in the original Japanese becomes a Spanish teacher in the English dub, which is roughly equivalent in terms of the role they'd play in a character's life (they teach a language most people are familiar with but might not actually speak fluently).

Sometimes it's even inconsistent within a work. They list a French dub of Pearl Harbor that translates English to French, but leaves Japanese as Japanese (which would align with the experience as created for English-speaking audiences, who would have understood the English bits but not the Japanese bits).

See also their pages on cultural translation or Woolseyism, where a work isn't even translated directly, but is instead translated to keep the same feelings, sense, or audience reaction.