LogTextLocalizationManager:Verbose: An attempt was made to get a localized string (Namespace:gameplay, Key:gameinstance, Source:hello), but it did not exist.
Did you build the .locres files? The only files that the engine actually reads are the *.locres files, so if you didn’t generate them then they didn’t make it into the engine.
I have found that this issue was related to missing config in DefaultEngine.ini.
I had to put localization directory there under [Internationalization] category. Now everything looks ok.