When you want to translate 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 of the default language which you fill in with translated content.
Select the Language Comparison function in the upper menu bar of the Web > Page module. Then use the select box to create a new translation of the page:
The form with the fields for the translated page will then be displayed:
Most of the fields are the same as for the page of the default language. Below the fields the content of the default language is shown.
If a page has no translation (that means, no "alternative page language" record), its content cannot be translated, no matter how many system languages have been defined.