Japanese band, but the song is 100% in English. There are versions of this video available with subtitles in other languages, including Japanese, but the one I linked has English subtitles. AFAIK there isn't a base, "no subtitles" version (though there should be, so others can make their own subtitles).
They wrote the song for the nuclear power plant disaster in Japan, but they wrote it so it could be enjoyed and its message received by anyone suffering from any kind of tragedy, of any size.
The song's message is simply, no matter what the darkness is, be the light (for others). The last part of the song, which is spoken, goes, "Some days just pass by, and some days are unforgettable. We can't choose the reason why. But we can choose what to do from the day after. So with that hope... with that... determination... let's make tomorrow... a brighter, and better day."