Database: tables for ArchWiki-specific page language association

[lahwaacz]

This would be basically a parser-like cache (#42) because the page titles would have to be parsed. It would be useful for extending db.query with language filters, e.g. generator="allpages", gaplanguage="en".

Note that the page naming scheme may change, see Language namespace(s) in place of suffixes?.

[lahwaacz]

Maybe this will not be necessary, the page table has a page_lang column and even MediaWiki could use it for something useful... See Incorrect metadata and RTL style issues.