Factory defaults are limited to one language. This means that in order to switch from one language to another, you must load new factory defaults to your database.
The UI literals are not a part of the factory defaults. They can be switched from one language to another as required, as long as they have been translated appropriately.
When using an Oracle database, it is possible to use Japanese characters in English factory defaults. In fact, it could be any character in valid Unicode and UTF-8 range.