I created a custom language file from english.txt, and when I tried it all the text strings were wrong. Turns out there are blank lines at random intervals in english.txt which were throwing the text strings off. I deleted the blank lines from my custom file and it was all right again.

Why did the blank lines affect my custom file and not english.txt?

english.txt has not one single blank line in it.

Oh, okay...seems my primary text editor Docpad is at fault, as I opened it in NOTEpad and found you were right.

notepad all the way :)