Even though you’ve placed this rule at the top of the .htaccess file, RedirectMatch is a mod_alias directive so still runs after other mod_rewrite (ie. RewriteRule) directives, so you may have a conflict.
Try changing this to a mod_rewrite directive. For example:
RewriteRule ^language/(.*) /members/?members_search=$1 [R=301,L]
NB: No slash prefix on the RewriteRule pattern when used inside .htaccess files.
You will likely need to clear your browser cache before testing. Preferably test with 302 (temporary) redirects to avoid potential caching issues.