specified as a ISO639-1 (two-letter) language code.
* "`currentlang()`" - tests whether a page is written in the same
language as the current page.
+* "`needstranslation()`" - tests whether a page needs translation
+ work. Only slave pages match this PageSpec.
Note that every non-po page is considered to be written in
`po_master_language`, as specified in `ikiwiki.setup`.