Filipe,
That definitely sounds like the right file, if you're language is set for en-GB. If your Joomla language is set to something else, you'll need to edit that file.
If that doesn't help, can you let me know what text you're trying to change? Another thing you may want to look into is setting the "Debug Language" setting in the Global Configuration area of Joomla. Then, when you view the page, any translatable text will have * characters around it, or ?? if there was no translation found in any of the language files.
Hope that helps get you started,
Alex