It's the proverbial pain in the backside if you don't have the right keyboard to go along with the characters you want to type.
A poor workman always blames his tools.
I've got a UK keyboard on my computers, and yet have no problems whatsoever compiling lengthly missives in French should I feel the urge to do so and can type in French at the same rapid pace that I can English, and certainly a lot quicker than if you put me infront of an AZERTY keyboard.
Practice makes perfect, its simply a case of knowing the shortcuts. Its a lot easier on a mac (option + e, option + c etc. etc.) than on a PC where you have to do stupid tricks with alt and numbers.
Sure Chinese might be a different matter.... but here we're just talking about languages that use just a few extra diacritics on top of the standard alphabet.