If I set the user interface language setting "Autodetect", I found it doesn't work at all and I should only manually change it to "CN" (Chinese). In order to find the problems, I make a simple debug on Linux: First set language setting to "Autodetect", then run command "./ugene -ui" in a terminal. Surprisingly, a warning happens, showing "Translation not found: transl_zh_CN".
That's just the problem! I've checked other QT applications like goldendict, its translation files were named as "cs_CZ.qm", "zh_CN.qm", "ru_RU.qm", while translation files of UGENE were "transl_cs.qm", "transl_cn.qm", "transl_ru.qm". If we change to "transl_zh_CN.qm", then perhaps it would work.
"Autodetect" settings doesn't work on my Windows OS, either