In Scrabble, both American and British spellings are accepted, because both are equally English. The number of "foreign" words in the English Scrabble dictionary is mind-boggling. Anything that has anything to do with South Africa, India, or the Philippines is going to be in the dictionary, since English is an official language in all of those countries thus those words would be used in those countries' English. Every country's words for their monetary units (old and new), administrative divisions, geographical terms, food items, political offices, etc. may or may not appear in the dictionary.
I find the arbitrariness of the dictionary used by my Scrabble CD-ROM to be a bit of a nuisance-- I've had numerous words (that really should have been valid) challenged off the board by Maven, many of which would have been bingos, including POUTINE (a classic Canadian food item). It also rejected GOUDA, the classic Dutch cheese. Maven's acceptance of words prefixed with RE-, UN-, and OUT- seems really arbitrary as well-- I learned that REAIRING isn't valid. Yet that's exactly what a TV show is doing if it's being shown for the second time. It's also very choosy about verbs converted to nouns with the -ER suffix.
I know the rules are different in other parts of the world but in the US, if you are in a tournament setting and you play an invalid word and it is challenged by your opponent--
or you challenge your opponent's word and it
is valid-- you lose your turn and cannot exchange any of your tiles (you must play the next turn with the same tiles). I like this rule because it gives players an incentive to actually know the dictionary-- you can't just "guess" your way through the game by playing a bunch of words of questionable acceptability until one of them is accepted, and you can't challenge the other player's perfectly good word with impunity. You're punished for not knowing the dictionary (or at least not knowing it as well as the other player) so players have to, you know, actually study the word list. Buy an official Scrabble dictionary-- you can get them at any bookstore or even Wal-Mart-- and hope you have great memorization skills.
The only problem is, my dictionary, and my CD-ROM, are both over 20 years old now, so today's word list isn't quite the same one I grew up with. I disagree with the decision to add stuff like QI and ZA to the list-- certain letters (C Q V Z, in the '90s dictionary) are great for "closing up" a board defensively and limiting bingo opportunities, as no valid "twos" (2-letter words) contain those letters.
Remember, folks, a good Scrabble player always
RETAINS certain tiles whenever possible!