[BUGFIX] change Site.lang to Site.code

The i18n files use WMF language codes instead of ISO 639 language
codes. This means we also have to use these in our translations.

e.g. site code of the Alemannic wikipedia is 'als' whereas the
language code is 'gsw'. The i18n files use 'als', while we now
try to lookup 'gsw', which does not exist.

Change-Id: I3bd186c06ef3b0506411f944f36f1b999fb35dfe
Merge "[BUGFIX] change Site.lang to Site.code"