in your bitcoin.conf file insert
lang=en
and restart bitcoin-qt
Thanks for the help Psy, having a look, looks like the latest version stores that somewhere else on OSX, checking this out at as I'm not finding the config file just at the moment.
https://en.bitcoin.it/wiki/Running_BitcoinYou could also set the language on start from the command line. Just append
-lang=en
to the normal command to start bitcoin-qt
Also, the OSX data dir is ~/Library/Application Support/Bitcoin/
https://en.bitcoin.it/wiki/Data_directory