Standalone: language change does not stick

To reproduce:

Administer > System Settings > Extensions: Enable the update language extension, saves time, if you haven't already downloaded the translation files
Administer > Localization > Languages: Enable two languages (no need for multilingual, just enable a second language)

You should then be able to display pages in different languages, using the lcMessages=xx_YY parameter. Ex:

https://crm.example.org/civicrm?lcMessages=en_US
https://crm.example.org/civicrm?lcMessages=fr_CA (adapt to the locale you enabled)

This works for a single page. However, CiviCRM should normally save the language in the $session object. See CRM_Core_BAO_ConfigSetting::applyLocale. It does not seem to be happening.

(I'll try to circle back, just wanted to log my findings so far)

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