I'm so far a fairly infrequent visitor to this site, just trying to keep my few plants alive, but thought I'd offer an outsiders perspective on this, from a ♦ over on SU.
This jumps to mind because it's something that we've had a fairly recent discussion about over on SU, but there's also a more general post on Meta.SO if you want some wider reading.
Unify your tags under one form and use synonyms where appropriate. SE sites seem to gravitate to the American English spellings or names where there is a difference, so it's probably best to stick with this.
Let people write their questions (body and title) in whatever variant of English they like - American, British, Canadian, Indian, etc - as long as they can manage the expected level of spelling and grammar, you shouldn't try to force any specific variant on a user.
It doesn't matter that SE is based in the US or that whatever proportion of visitors are from the US - especially when a site is so young - because changing posts between language variants unnecessarily will lead to edit wars, comment arguments, meta posts and will drive users away. It will ultimately be very bad for health of the site, because unhappy argumentative users are toxic.
However, where there are large cross-language differences (such as completely different names for things), then Eight Day's suggestion of inserting an in-line explanation of the term the perfect solution - you are then augmenting a user's post, making it better for everyone, and not unnecessarily mutating it.