When set the current asset group language in a particular process, the asset refers to the "source asset" and output warning log.
The following code is a workaround for updating the cache of the asset group when changing the localized language.
Adding update asset group culture cache when changing the culture.
1. Open attached project in editor.
2. Packaging project with Win64.
3. Launch package application and wait around 5 sec.
(automatically process in StartupMap)
SetCurrentLanguageAndLocale(ja) -> SetCurrentLanguageAndLocale(en) -> SetCurrentLanguageAndLocale(ja) -> SetCurrentAssetGroup(en) -> OpenLevel(LocalizationMap).
4. After moving level, press Langugae button.
Play sound source asset.
Play sound localized asset (en).
The Asset Group (Audio) is set to "en". So expect "en" in the Asset Group to be selected even if the current language is "ja".
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-103591 in the post.