Create cy_GB.lang

Original: https://github.com/Lapiman/Equivalent-Exchange-3/blob/master/resources/mods/ee3/lang/cy_GB.xml

Updating the Welsh localization to the new format. Similar pushes will appear soon.

These are simply copy-and-pasted from the old localizations I have from 9 months ago; new stuff is left in English. I can't figure out how to view the authors- someone well-versed in git, mind looking that up for me and giving some credit where it's due?