Possible alternatives:

• Rescue the old code from Translate and copy to TranslationNotifications (probably easiest)
• Use the new code from Translate (creates OOUI vs. non-OOUI mismatch in looks; I think it isn't a big deal)
• Convert the whole form to OOUI (most amount of work, but would bring some consistency with rest of MediaWiki)

Change 625637 had a related patch set uploaded (by Abijeet Patro; owner: Abijeet Patro):
[mediawiki/extensions/TranslationNotifications@master] Fix broken language selector on the Special:NotifyTranslator page

https://gerrit.wikimedia.org/r/625637

For now took this approach: Rescue the old code from Translate and copy to TranslationNotifications (probably easiest)

Did not see a straight forward way of using just the new LanguagesMultiselectWidget for this control, since HtmlFormFactory expects $displayFormat to be either a normal vform or ooui.

Change 625637 merged by jenkins-bot:
[mediawiki/extensions/TranslationNotifications@master] Fix broken language selector on the Special:NotifyTranslator page

https://gerrit.wikimedia.org/r/625637