Listen to Kim Da-Hee (or 'Dahee Kim' in our convention) who is SeeU's voice provider perform this song in virtually fluent English. The present technique of fudging it from her Japanese and Korean banks of phonemes works reasonably well, but has notable deficiencies.
Listening to Ms Kim here, one is struck by both the similarities and differences between her natural voice and SeeU's, the latter sometimes coming across a little lacklustre. Apparently this was a mistake of some sort made when the voicebank was originally recorded, and is often compensated for by having her provider add a (quieter, but effective) duet-style enhancement.
The overall effect is very distinctive, but it is a pity it has to be done this way. Hopefully, if the English voicebank does go ahead, it will be done correctly this time.
I have read that there are indeed real hopes for an English voicebank for SeeU, perhaps even later this year, though all of this is unconfirmed. It is now obvious that her voice provider is fully capable of supplying what could easily be the best (foreign language origin provider) English voicebank in the world. I'd say to her owners: Go for it!