Create TranslateSvgUtils::langCodeToOs() and osTolangCode()

Although not particularly complicated at the moment, could easily
become so in future as more quirks are discovered in the way
systemLanguage is interpreted.

Change-Id: I1599c25c8756e96b5da4b036289b6b5c882b154c
Merge "Create TranslateSvgUtils::langCodeToOs() and osTolangCode()"