After installing the English version (downloaded anew from the official site), reboot, and go to avast main GUI -> settings -> language -> select the language you want -> install -> save settings -> reboot -> test and report back.
So I did, After rebooting my pc (because I had tried, but failed to remove avast, I had to reboot) I got a message in French(??) that my product key had expired. I ignored it since I was going to reinstall.
(Now that I had Avast opened, I could deactivate the auto-protect, but didn't)
I reinstalled the English version, withou any problem.
I rebooted and got the notification again, in French.
When I opened Avast it was partially in French, partially in English.
I installed the Dutch package and rebooted.
After rebooting i could not select another language in Avast.
I reinstalled the Dutch package and then I could select it as A language, no reboot.
I renewed my key right after.
Now it seems to be working just fine, in Dutch.
Thanks again for the help!