Dear Chaley,
1) I have set the UI to Vietnamese and I don't change it back to English, but sometimes most (not all) of the UI is reversed to English, and later it will be in Vietnamese again. It happens in all of my devices, running Android 6.0.1, 5.1.1 or 4.4.2, so I think it is not a system issue. It seems kind of random. I know it is weird.
2) The "Languages" under Grouping Drawer: It's the default Language field of Calibre. In my PC, Calibre is in English UI. However, other fields are automatically translated in CC. I have checked in Onesky and all "Languages" have been translated.
As you can see in the screenshot, it is when the UI is reversed to English. And the "Languages" under Drawer as well.
//EDIT: Under the "Groups from custom colums" dialog, I can see Formats (formats), Languages (languages), Date (date), Published (pubdate). They are the default Calibre fields, not custom columns.