For communication sites, the language of the access request is the base language of the site (note, not necessarily the site collection). It does not depend on the language to the requester nor the language of the recipient. The base language is set when the site is created, and can not be changed later.
Most versions of Outlook let you translate most messages. Unfortunately, these types of system messages are not well handled by Outlook's translation, so only small bits of it will be translated by Outlook. Even Immersive Reader is unable to translate much of it. You're stuck with messages you don't understand, unless you delete the site and re-create it in a different base language. Or forward the message to a gmail account, read it on a browser and use Google's translation.