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When switching localization for testing, text does not change language in PIE.
1. Open UE4Editor (any project)
2. Add UI folder
3. Add widget blueprint inside UI folder
4. Add text block to widget (save the text entered)
5. In Editor Preferences->Experimental, check the box for Localization Dashboard
6. Open Window->Localization Dashboard
7. Click Game target and check the box for Gather from Packages
8. Expand Gather from Packages dropdown and Include Path Wildcards dropdown
9. Click the ellipsis and add the UI folder
10. Under Cultures click Add New Culture and select Russian
11. Set English to native culture
12. Click Gather All under Targets heading
13. Click first icon in Russian culture Actions (Edit Translations)
14. Use google to translate saved text and paste below Choose Translation Font then save
15. In Localization Dashboard click Compile All under Cultures
16. Click PIE in main editor viewport
17. Open console command window and enter 'culture=ru'
Text in the widget will remain in English
Changing culture changes the language of on screen text. This does work in a standalone game rather than PIE. This also worked in 4.8.3
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-20977 in the post.