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FWIW the typical phrase is "en masse" (which is French). The English translation is "in mass".

The English translation would be something like, “all together, at the same time”. “In mass” isn’t an English idiom.

"en masse" is the term used in English as well.

Yep, just noting for anyone who may not realize that "in mass" isn't really a thing in English.

As you note, en masse is common — enough so that you even don't need to italicize it (although you can).

If you're brave enough, you can italicize whatever you want.

Indeed. Tips on when you should italicize foreign words in English writing: https://proofed.com/writing-tips/should-you-italicize-foreig...