I just ran into the EXACT same issue as you, Cadiboo. By which I mean that I'm almost 100% sure I know what was causing YOUR issue, for 2 reasons. Posting this for anyone else who comes along.
It was your encoding. Need UTF-8 ...WITHOUT...BOM.
I was getting unlocalized output...
...so I tried changing my lang file from "UTF-8 BOM" to "UTF-8" (no BOM)...
...and now it works.
This also explains the 3 byte difference in file sizes, because BOM adds 3 bytes of data to the file header.
Hopefully this saves someone else a bit of time.