In addition to choosing a different font or as an aside, you could also utilize what comics do to indicate someone is speaking in foreign language which is with this <>
So it would look like this:
<I am speaking in a foreign language right now>, and now I've switched back to English/Japanese
Looks like these panels below. And if you have a multi-bubble speech bubble, then each individual bubble has the <>
Just another option for you in case you wanted something else to distinguish when a character is speaking in another language