From my experience with several multi-lingual Grav installations, this is not a problem with Grav.
As a non expert, I have had lots of luck with .htaccess Redirect Generator - Daniel Morell. Give it a shot or two.
I also remember that such directives, within your .htaccess, must be located at the top, before Grav’s own directives.
You may test with https://htaccess.madewithlove.com/.