Cyan148 It's technically against EA's terms of service for one person to have multiple accounts, although I don't know that it's enforced much. But you don't need two accounts for this. You can change the language for Sims 4 within the EA App, under Manage > View properties. When you select a new language, the game should start repairing, really just downloading the new language string files. If the repair doesn't start on its own, you can do so manually.
Most languages name the user folder "The Sims 4," but four of them (Spanish, French, German, and Dutch) have localized names, e.g. "Los Sims 4" for Spanish. If you're using one of these languages, your saves and other user data won't overlap with the other language. You'd need to copy your saves, Tray files, etc. back and forth to be able to use them in the other language.
If neither language has a localized name for the user folder, then all the files will be in the same place. Some subfolders will have localized names too, but that typically doesn't matter, and you can always copy around files if you decide it's necessary.
Either way though, names of some places and sims will be localized and will NOT change within a given save even if you load that save in a new language. If this bothers you, keep your saves separate. It's fine to have saves from multiple languages in the same folder, or Tray files, or any other user files. You won't break the game in any way.