Add i18n translate alias function to bootstrap

Related to https://github.com/kohana/core/issues/526

When implementing I realised that I18n::lang() had to be set for the function to be useful, so in the end it did have to be placed after Kohana::modules().
I left it in because the index.php doesn't explicitly say that the bootstrap should only be included once https://github.com/kohana/kohana/blob/3.4/develop/index.php#L103 and there are existing checks for other globals https://github.com/kohana/kohana/blob/3.4/develop/index.php#L89.