Choose a spelling system, American spelling or British spelling or any other variety.
If you want to use British spelling: Consider using -ize instead of -ise (Oxford spelling). Both are correct in British English. The suffix -ize is more international, where -ise is more British. Consider dropping the letter 'e' is some words: judgment instead of judgement, aging instead of ageing.
If you want to use American spelling: Consider using double l's in words like traveller, labelled, cancelled. Consider using theatre, glamour and grey instead of theater, glamor and gray.
Use abbreviations if possible to avoid the need of choosing spelling alternatives: 14 km instead of 14 kilometers/kilometres, 25.4% instead of 25.4 percent/per cent.