I think this is really a non-issue. Any decent search engine will return hits for your paper, even if one searches for harmonisation (the spelling more common in Britain) rather than harmonization (the spelling used in the USA, and also valid, though less common, in Britain) as long as a literal search isn't used.
I'm amending my answer to address the part of your question about whether specific journals require certain spellings. A Google search limited to the .edu domains finds lots of author guidelines that address this issue to various degrees, but nothing shows me that there's a single way to go.