The language selector is displayed on all pages at the top right. Existing translations are shown in blue, missing translations are gray.
Only languages manually configured by the administrator are shown in the selector. If you want to add translations for a language currently not shown, just manually create the pages in the appropriate language subnamespace and send a mail via the Contact Form asking for your language to be added.
Where translation is not sensible the ~~NOTRANS~~ macro should be inserted into the page.