No entity in module system can get changed. Neither can any of system’s entities be made owner of a new association. Your only option is to create an entity in one of your own modules and associate it *-1 to system.language. 1-1 and 1-* association is not allowed. If you need it to be a 1-1, add a BeforeCommit and/or BeforeChange microflow to ensure this. You will still suffer from the *-1 association though when retrieving objects from your entity via System.language, since it will return a list.
Thanks for your comment. Then I have to use a *-1 assosiation.