Reproducible on 4.6 and 5.0 versions;

Repro steps:
1. open project "WinLang.zip";
2. build and run project to WSA platform;
3. run the project in the editor;
4. go to "Control Panel -> Language" and add or change the order of the first keyboard input language in the list. Observe how changes affect WSA player and editor.
If you change region in "Control Panel -> Region", editor will show updated system language according to region, but Win Store Apps will not be affected;

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By design:
It seems in Windows Store Apps it's not possible, as there's no API to get that value.

If you like to explore this issue more, check out this link:
http://stackoverflow.com/questions/12560412/winrt-apps-and-regional-settings-the-correct-way-to-format-dates-and-numbers-ba

It contains answer from Microsoft employee with explanation.