But try (in Word) Tools --> Language --> Set Language
I have a box ticked called 'set language' if your PC is in Welsh as default then maybe it's picking up on that, so try unchecking it. You may also want to chec that your PC is not set to Welsh. You can do that through My Computer --> Regional/Language settings.
If it's still reverting to Welsh you can try setting your default document template (normal.dot) to English by opening it, changing the language and then overwriting it.