symfony/symfony-docs — comment on commit 1718f04e19452b6c89855762b385d10b93380c7e
File: book/routing.rst line 791
By weaverryan:

not true - if there is a `_locale` request attribute (i.e. it's been set on the route), then it *is* set on the session:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/EventListener/LocaleListener.php#L45

but if I'm reading it wrong, let me know :)