add method converting language string to 2char code
passed language string could be either other code or full language name
(cherry picked from commit 4b9cc5daded526d70ec2def1fae4aeedb83bb65f)
Add doxygen comment for new methods in LangCodeExpander.h and change param order.
These are ConvertToTwoCharCode(...), CodeToString(...) and ReverseLookup(...)
Change parameter order of ConvertToTwoCharCode(...) to be in line with the other convert methods.
[Python] Add xbmc.getLangauge(int format = 0).
Allow to specify the format of the returned string, possible values are:
0: full language (in English)
1: two letter code (ISO 639-1)
2: three letter code (ISO 639-2/T or ISO 639-2/B)

The default value is 0.