add language detction/swapping mechanism

[struan]

We need a way for user's to switch to, in this case, Welsh from English. Our other sites to this using a subdomain so that should be fine in this case.

So this should:

• detect if we are using cy.wtt as a subdomain
• if so set the locale accordingly

I don't think we need to persist this between visits, especially as initially repeat visits will be from links in emails where this should persist.