I have documents in Chinese and English, and my OS is Ubuntu 18.04 English, I could install Chinese Simplified and Traditional with no problem, according to the documentation, anything below English is ignored, and my both Chinese are below, so I tried to move them up, the trouble is no matter how I tried, I can only put Chinese above English US, and my display preferred order is English US, Chinese Traditional and Simplified, whenever I tried to put English US above Chinese, both Chinese went down to the bottom of the language list. Is this a bug? Am I missing steps to put in the order that I'd like to? TIA
That's intentional behavior. If you think about it, English is the original language and hence always available. Accordingly, non-English fallback languages would not make sense.
If you open Language Support and click the Help button, you'll find a help document which among other things explains the language priority list.
Please note that the set language(s) affects in which language various menus and messages are showed. It does not affect the ability to display documents in other languages.