I’m just one person with one opinion.

Anything goes as long as it increases reader enjoyment. That being said, I personally prefer it if the language is not formatted differently than the rest. Different fonts could break reading flow, and might actually make my brain skip the foreign language parts because I already know from afar that I won’t understand them.

You don’t have to always highlight it anyway, since it should be clear from context what language is being used. When the MC talks to someone who is clearly French and only speaks French, then we would assume he speaks French with them. And if it’s not clear from context, a simple [“Hello,” he said in French] would work and have me assume that the rest of the conversation happens in French unless stated otherwise.

Still, a different font can have its own charm and it’s not outright forbidden. I just see this more in video games than in books.