Reciprocal fallback between ca (Catalan) and oc (Occitan)

Reciprocal fallback between ca (Catalan) and oc (Occitan)

The two languages are extremely close, to the point linguists have
considered one a variant of the other till not too long ago. The
two linguistic communities have strong ties according to Donneky,
2011: http://hdl.handle.net/10443/1302 . Speakers should have no
difficulty understanding each other.

A translator has compared a sample of translations in the two
languages and found them to be largely similar and compatible.

Use European date formats; no $namespaceGenderAliases for now in oc.

Discussed at:
https://translatewiki.net/wiki/Thread:Portal_talk:Ca/Joining_forces_with_Occitan

The proposal has been notified on all Catalan Wikimedia projects, see
* https://ca.wikipedia.org/wiki/Tema:Tjc8xr0z769mskl5
* https://ca.wiktionary.org/w/index.php?diff=903923
etc. has met a very positive response.

This change is not incompatible with the oc→fr fallback (I02fc9af984).

Change-Id: Ibbad7a830bc010654db8ad5679d6a0f45357d382