Misspelled words rob documents of authority. With built-in spelling checkers on tap such mistakes are heinous. Readers will reason that if the author doesn't take the document seriously, why should they? The next thought that's likely to occur to them is that the program has probably been tested with the same lack of professionalism.
If the product is to be marketed in the United States, use American-English spellings; for other English-speaking markets, use world English-English. The differences are subtle and few. The American spellings were created for phonetic consistency by Noah Webster in 1783 (The American Spelling Book 260 impressions from 1783-1843). In the intervening 200 years, we have grown to recognize both spellings. If you can't run to two print runs, use the localization for which you believe most of the product will be sold.
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