- Stack Exchange generally uses American English rather than Commonwealth English.
- People from Japan use American English rather than Commonwealth English.
Usability issues can appropriately be solved using tag synonyms...
As for re-tagging in order to fall in line with the official guidelines... Sure, I guess (more on account of not caring all that much, than on the strength of the OP's arguments)...
(Needless to say, spelling flavour in posts does not move and should never be edited. But I'm sure that was clear already)
I prefer tags in American English to make them look consistent with other part of user interface of the website, but I do not care much. But either way, the other spelling should be a synonym for the real tag.
I don't find Japanese to use more American English at all. In fact English borrowed into Japanese overwhelmingly takes the not-rhotic pronunciations to Japanese rather than rhotic ones. This means "r" sounds following vowels in English words are generally omitted when they become Japanese words even though it's perfectly possible to preserve them. Although there are non-rhotic American accents such as Boston and rhotic Commonwealth accents such as Scotland there is a strong marker.
I find changing between correct spellings to be disrespectful to the original author. I prefer a policy like on Wikipedia where both spellings are fine and whichever one was used first should stand. Synonyms can take care of the rest.