Set Czech (cs) and Slovak (sk) as reciprocal fallbacks

Set Czech (cs) and Slovak (sk) as reciprocal fallbacks

The two languages are mutually intelligible at a very high degree and
most speakers can be considered bilinguals or passive bilinguals.

Consulted resources:
* Eurostat special eurobarometer 386 (2012).
  http://ec.europa.eu/public_opinion/archives/ebs/ebs_386_en.pdf
* Mira Nábělková
** http://doai.io/10.1515/IJSL.2007.004: «These differences do not prevent the users of Slovak and
   Czech from understanding discourse in the other language [...]. While contact with Czech texts
   in the Slovak environment is obvious, it does not appear to be so in the opposite case.»
** High degree of passive bilingualism even in recent generations who don't feel it much.
   https://repozytorium.amu.edu.pl/bitstream/10593/9168/1/02_Viera_K0VA%C4%860VA_Czech_Slovak_intercultural_265_280.pdf
* Discussions with several translators.
** https://translatewiki.net/wiki/Thread:Portal_talk:Cs/Fallback_to_sk
** https://translatewiki.net/wiki/Thread:Portal_talk:Sk/Fallback_to_cs

Change-Id: Id85b281a3ef2f2ae00ee0aba827e94e4f5a1f924