Hi Megan,
Multi-language sites are indeed tricky to manage. I have a question - do you have any view on how good Google Translate it at converting English to Welsh?
You could see how effective it is by going to https://translate.google.com/ and selecting Websites, then entering the URL of your site (or a page of your site) to see how good it is.
If it's good, then one option would be to add a Google Translate widget to the site, so users can select Welsh (or indeed any other language) if they wanted to, and it'd automatically translate from the English pages on the site.
That way you'd only need to maintain the site in English and the Welsh would look after itself.
Try it out and let me know if this sounds like a good option!
Thanks
Joe