The French keyboard is the most notable example of anyone using something other than query or quertz. Even Japan and China use an extended querty. But even with the French keyboard the only issue is that everything is in the wrong place, not that the standard 26 "English" letters don't exist or are hard to reach.

Meanwhile using ä, é or ş in a username or password will make your life much harder once you are in a foreign country. Never mind any letter that isn't derived from the Latin alphabet.