See Paul Edstein's response here:
https://www.msofficeforums.com/word/...hey-wrong.html
If you are comfortable with using or willing to work with macros something similar could be done.

Do you want them to be Italicized or simply to not show up as errors?

If you wanted to both Italicize the words and have them not marked as errors, I would recommend using a character style that has both the language attribute and Italics.

I suspect the best way for you may be to have a macro go through each spelling error and ask you whether you wanted to mark that as the other language and Italicize. That would take more time than just marking all errors as the different language.