I have implemented a site with several languages just as described in the tutorial. But when trying to display text using the _t() function it always is displayed in the default locale language (spanish). I managed to get as far as finding out that this happens because _t() gets the locale from i18n::get_locale() and this function for some reason always returns the default language.

Just for testing I made a function in my Page.php that returns a string with the values of:
- i18n::get_locale()
- $this->Locale
And Bingo! On the english version of the page the first function returns es_ES while the second value is en_GB. On the spanish version both return es_ES.

Is this normal? Am I missing something? Can _t() not be used this way?

I fixed it by adding
i18n::set_locale(Translatable::get_current_locale());
to the init() function of the Page_Controller.

As pointed out in this post http://silverstripe.org/general-questions/show/12186 this behaviour is a little strange and IMHO it should be enabled by default because its not clear in the documentation and being able to use _t() in the Template is kinda stupid when it doesn't work.

If anyone has a nice explanation why this works this way please reply anyway.

Thanks for this tip, it helped me out.

This should be working by default, you're right about that.