Google Translate sounds like it might be an option, although as said above the translated pages won’t be in any search index. This will add a bar to the top of each page offering to translate it into Welsh.
Welsh is a pretty minority language though (most people who speaks Welsh also speak English do they not?), so I’m not sure what benefit you’ll really be offering here.
Sorry maybe I wasn’t clear, have them translated (machine, human, whatever) and then create the new pages rather than having a translate option on the page is what I meant.
Google Translate will do an automatic translation (if you redirect the welsh users to the translated version when they load the page - perhaps using PHP). But be warned… automatic translation is no-where near as accurate as manual translation, if you want the job done properly, get a professional in to translate everything.