How do I translate text in templates ?

Here's what I've done (With Martijn's help via IRC) :

in Page.ss

<% _t("Page.READMORE","Lire la suite") %>

in mysite/lang/en_GB.php

<?php

/**
 * English (United Kingdom) language pack
 * @package sapphire
 * @subpackage i18n
 */

i18n::include_locale_file('sapphire', 'en_US');

global $lang;

if(array_key_exists('en_GB', $lang) && is_array($lang['en_GB'])) {
	$lang['en_GB'] = array_merge($lang['en_US'], $lang['en_GB']);
} else {
	$lang['en_GB'] = $lang['en_US'];
}

$lang['en_GB']['Page.ss']['READMORE'] = 'Read More';

?>

and in_ config.php :

...
// Set the site locale
i18n::set_locale('fr_FR');

// enable nested URLs for this site (e.g. page/sub-page/)
SiteTree::enable_nested_urls();

Translatable::set_default_locale('fr_FR');
Object::add_extension('SiteTree', 'Translatable');
Object::add_extension('SiteConfig', 'Translatable');

Thanks to Martijn and Zauberfisch on IRC Channel, here's what I missed :

in Page.php

public function init() {

    parent::init();

    if($this->dataRecord->hasExtension('Translatable'))
    i18n::set_locale($this->dataRecord->Locale); // set i18n language for _t() in template
}

If you have Translatable enabled, then a call like this (in init() should be sufficient:

i18n::set_locale(Translatable::get_current_locale());

It's basically the same as you do now, just shorter ;)