Is there any software to regularize British vs. American spelling in a text? This is a minor but ongoing annoyance that takes up more time than I would like, and I am wondering if anybody knows of a solution.  I often write papers with foreign collaborators, and we tend to end up with a mixture of British and American spellings of words in our text.  We know it's a problem, and often agree on which to use ahead of time, but it's so ingrained one often doesn't even realize when you've made a mistake.  Thus, in the final proofing stage, somebody has to go through and manually regularize the spelling, which is really boring and time-consuming.

My question is this: does anybody know of software that can help automate this process?  It feels to me like it should exist, but I don't know of any...
Yes. There is an add-in for Microsoft Word called myWriterTools that will convert US to UK English and vice-versa. It also highlights changes so you can double-check. It isn't free however.

Alternatively you could use CTRL + A to select all your text in the existing document and set the proofing language to your desired version of English. This will then show the majority of errors as incorrect spelling (be wary of words such as license and licence though which may be used in different contexts) in Word.

Compass in a comment has mentioned an online converter US2UK that may be useful for shorter documents.

I have searched for a LaTeX equivalent but it would appear one does not exist currently.