The other day, one of may translation colleagues wrote about this in her blog. I used to think, and according to her blog, she also believed, that ファーストフード is the correct way of writing “fast food” in Katakana, but apparently ファストフード is being used more and more often these days in Japan. As far as the sound is concerned, of course, ファストフード makes more sense!
A similar but more obvious change is from ミルクセーキ to ミルクシェイク. When I was a child, “milk shake” was called ミルクセーキ in Japanese but when マクドナルド came to Japan, their milk shake was called マックシェーク (I thought it was spelt like this though I was too young to remember for sure) or マックシェイク and now ordinary ‘milk shake’ is called ミルクシェイク! Again, ミルクシェイク sounds a lot closer to the English “milk shake.”
Languages are changing all the time. Japanese is changing all the time, too!
If you know more of these examples, please share it with me as a comment below!