Add Welsh (UK) ISO language code

[NLW-paulm]

I note in the latest version of Madoc the language code for Welsh is cy, however a lot of the manifests we have imported have identified that language strings for Welsh cy-GB were found in content we imported. (Welsh UK) So we cant align/add this with our Core welsh language translations.
Would it be possible to either replace the Welsh language cy folder with one called cy-GB, or add an aditional language as Welsh (UK).

We can create a new pull request with the core language file in this folder too.