Current user's display language is changing automatically while editing other users

• Type: Bug
• Status: Closed
• Resolution: Fixed
• Affects Version/s: 6.0.6 GA, 6.0.12 EE, 6.1.1 CE GA2, 6.1.20 EE GA2
• Fix Version/s: 6.0.X EE, 6.1.2 CE GA3, 6.1.30 EE GA3, 6.2.0 CE M6
• Component/s: Legacy, Legacy > Enterprise Admin, User Management
• Labels:
  • 6.0_release_notes
  • QA-R
  • QA-TP
  • ee-ts
• Branch Version/s:
  6.1.x, 6.0.x
• Backported to Branch:
  Committed
• Story Points: 4
• Fix Priority: 4
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/11695

=== Steps to reproduce ===
1- Start portal with default settings
2- Go to Control Panel/Users & Organizations (or Users in 6.0)
3- Change your display language e.g. to "hu" by modifying the current URL: $yourHost:$yourPort/hu/...
4- Add a new user called "testDE"
5- Edit the Display Settings of this user: change the Language to "de"
6- Save

>>> Your current display locale will be changed to "de"

Note

• When your current display language equals to the portal's default one ("en"), the URL doesn't contain the i18n languageId & themeDisplay.isI18n() is not set, and the issue is not reproducible.
• If you set locale.prepend.friendly.url.style=2 the issue is always reproducible.