Tag synonyms are designed to be used when there are two different words with the same meaning. We don't typically use synonyms for things like regional spelling variations, pluralizations, misspellings, etc.
Once we establish which tag the system prefers, the text completion and tag wiki excerpts will help users find it. We tend to standardize on the US spelling for tag usage. Down the road a bit, it will require higher reputation to create new tags so this type of problem passes quickly.