I don't think this is a duplicate. The task this was merged into is about a behaviour which might be unexpected to end users, where the "log in" or "create account" links (especially, but also in effect all other links) don't get the expected uselang= parameter.

This task, however, is about what happens when you use ?uselang= in the edit form, and then a button like "Preview" or "View changes". For the tech-savvy who use uselang=qqx to identify which MediaWiki messages to fix, there is really no way to use that method to see which messages are used on the preview page, such as the message "Previewnote", which is shown between the edit field and the preview. Therefore I believe these tasks are two separate issues.

Pasting a comment Chico left in T135511, since it pertains more to this task.

In T135511#3046543, @Chicocvenancio wrote:

Since the tasks where merged, let me bring my comments:

I was trying to use &uselang=qqx method to check a few spamblacklist error messages. However, the uselang is not preserved after an edit submission, reverting to site default language.
In fact it seems to me the parameter is not sent on edit submission.

It does not persist as well on send thanks action. I was trying to discover what message was the one given after the send thanks action and luckly found it through translatewiki. (MediaWiki:Thanks-thanked-notice).
Uselang=qqx, however, proved useless.