Add word to autocorrect of a single language in Office

Question

I use multiple display languages in Microsoft Office - German as primary, English as secondary.

I would like Office to correct "im" to "I'm" when the editing language of the current document/e-mail is set to English, but not when to German (since it's a valid word in that language).

However, I can only see a single autocorrect dictionary in the Office apps, containing entries in my former primary display language.

How can I add this "im"-->"I'm" to only the English autocorrect dictionary?

Thanks for any help!