So, after some more research, I found the location for the unsubscribe/resubscribe messages and emails. They are located under the subscribers folder, while the others are located under the subscribe process folder...
However, the other question of mine is still open (email notification after resubscription).
I'm also wondering that the "new subscription" message to the system owner is in English language only and I didn't find where to customize it.
I would suggest to the development team to consolidate all this messages/email topics in one place to keep the process more elegant.
Thanks and best regards - and a happy New Year,
Klaus