When you want a translation of a page in the page tree you create an “Alternative Page Language” record on that page. This contains fields similar to that of the page record which you fill in with translated content:
Obviously, the most important field is the “Language” field. Most of the fields are the same as for pages.
If a page has no translation (i.e. no “Alternative Page Language” record), its content cannot be translated, no matter how many system languages have been defined.