Translation is done with a plot devices or a character translating. All foreign spoken languages are foreign. It's written in the first person, I believe, with strange switches to third person that tell plot lines the main characters would hear or might have heard about from other characters (this helps with avoiding exposition show not tell).
You can also go full ballistic and write the story without an omniscient narrator, like in Paper Girls.
It's established what language they speak, and since it's from their perspective, there's no need to use hyerogliphs ever again between the two.
In Asterix and Cleopatra two acient egyptians might have the first few bubbles in hyerogliphs, but two-three panels into the conversation, the bubbles switch to the main language if, like I've said, no gaul is there to witness. Switch the perspective from one character to another clearly. Even if it happens in the same place, one after another. Clearly separate into different scenes the introduction of a character who doesn't understand the spoken language from the conversion between characters that do. Just don't go introducing and removing characters to the scene that don't understand the spoken language. When you have a scene were all the characters understand the spoken language, don't use foreign language or different fonts at all. It switches to the narrator language when no gauls are present. In Asterix and Cleopatra, when the gauls witness a conversation between ancient egyptians the conversation remains in hyerogliphs. You can also switch to a different font (this also helps if you ever publish in different languages like it happens in the EU) when you switch into the foreign speakers' perspective. The character shouldn't understand, yet you still have an omniscient narrator. If it's first or second person, from a character's perspective that doesn't understand the spoken language, writing speach bubbles in the foreign language and using captions to translate is a good move. How you write foreign languages also depends from who's perspective are you telling the story.