Step 1. Add http://iphonedict.com/rep as an Installer.app source.
Step 2. Install a keyboard dictionary in your language. Found in category “Keyboard Dictionaries”
Step 3. Activate by choosing “English (UK)” in Settings -> General -> International -> Keyboards. The FAQ has more info.
Step 4 (optional). Delete all custom words you have typed on the keyboard: Settings -> General -> Reset -> Reset Keyboard Dictionary.
Questions? Read the FAQ.