[CharlesW]

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

[krapp]

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

[CharlesW]

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).

[capableweb]

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

[CharlesW]

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