[MediaWiki] Enable export for languages with an own Wikipedia

Those languages already a Wikipedia for historical reasons: if their
translations are not exported they get confused and unhappy and just
keep translating locally on their own Wikipedias instead of benefiting
all MediaWiki reusers.

Can't find a way other than a separate export command, because threshold
is always evaluated and unsets a language wherever it comes from.

https://translatewiki.net/wiki/Thread:Support/Enable_export_for_all_supported_languages_with_an_open_Wikipedia

Change-Id: I0007fd00579971d4d5bce5e4545492f87ea52867
Merge "[MediaWiki] Enable export for languages with an own Wikipedia"