Fix user preferences selecting the wrong preferred locale.

Fix user preferences selecting the wrong preferred locale.

Due to a wrong comparison, when loading the preferred locale in the
user preferences page, in cases like `zh_CN` or `de_DE` the wrong
locale would be chosen.

As with too many things, the code is duplicated on the `UserPage`
and the `EditUserPage`. And they differ. So extract the choosing of
the preferred language for display into a method in the (more up-to-date)
`UserPage` and call that from the `EditUserPage`.