Time zone label is not updated when selecting another language

The label 'Time zone:' in the login screen is not updated when another language is selected.

Steps to reproduce:

1. Enable the time zon selector in the login screen (see CMS-6954)
2. Open the login screen
3. Select another language

Expected: the 'Time zone' label changes to a translated version for the selected language
Actual: the 'Time zone' label does not change

A refresh of the login screen (F5) does update the label.