I agree fully on that, the thing is, a 1 week delay in Japan won't get you much, and the translating is probably done as the game is made, so I don't think it's the translation.
Personally I think it COULD be worry of shortages. Wii stock is very limited in US and Europe, but fine in Japan (I think) so this extra week is to make more DVD copies (after all, there weren't enough DVDs of Mario Galaxy...) and when you take into consideration what a game developer can do with 1 week... it's not much is it? If anything. This is why I think it's about shortages. Shortages gives Nintendo bad press, so whether you care about being able to wait for the next shipment, Nintendo do care, and they want enough copies to AT LEAST support the pre-orders (selling out on pre-orders is a bit of a problem...)
This is why they need more time in US, and there is a bit of a "game drought" for Wii at the moment (in US at least), so sales should slow down in the run up to Brawl and Mario Kart, before Nintendo release them and the selling goes crazy again. I'm hoping this delay will include a few tweaks and maybe character balancing, but there has been no announcement of going gold so who knows.