from PySide6.QtUiTools import QUiLoader
from PySide6.QtWidgets import QMainWindow
from PySide6.QtWidgets import QApplication, QMenu, QMessageBox
from PySide6.QtGui import QCursor, QAction
from PySide6.QtCore import QTranslator

class PP(QMainWindow):
    def __init__(self):
        super().__init__()

        loader = QUiLoader()
        loader.setLanguageChangeEnabled(True)
        print(loader.isLanguageChangeEnabled())

        self.app = QApplication.instance()

        self.main_ui = loader.load('test.ui')
        self.setCentralWidget(self.main_ui)
        self.main_ui.pushButton.clicked.connect(self.trans_ui)

        self.menu = QMenu(self)
        self.action = QAction(self.tr('hello'), self)
        self.action.triggered.connect(self.trans_code)
        self.menu.addAction(self.action)
        self.main_ui.pushButton.customContextMenuRequested.connect(self.show_menu)

        self.show()

    def changeEvent(self, event):
        print(event)

    def load_qm(self):
        self.trans_window = QTranslator()
        self.trans_window.load('test.qm')
        self.app.installTranslator(self.trans_window)
        
        self.trans_main = QTranslator()
        self.trans_main.load('ttt.qm')
        self.app.installTranslator(self.trans_main)

    def trans_ui(self):
        self.load_qm()

    def trans_code(self):
        self.main_ui.label.setText(self.tr('pear'))
        QMessageBox.information(self, self.tr('cat'), self.tr('apple'))

    def show_menu(self):
        self.menu.exec(QCursor.pos())

if __name__ == '__main__':
    app = QApplication()
    ui = PP()
    app.exec()

The related ts file:

There are a button "translate" and a label. Right click the button, a menu will appear. And then left click the 'hello' action, a QMessage box will appear and text 'pear' will be set to the label. I hope that before I left click the button, all the text are English. And after I left click the button, all the text can be translated to Chinese directly.
But in the below GIF:

Problem 1: after I left click the button, the action text 'hello' was not translated. The label text 'pear' was translated afterI left click the action. (I hope it can be translated as soon as I left click the button.)

If I change the code as follows, which means load 'ttt.qm' before load() ui file just as what @JonB said.

class PP(QMainWindow):
    def __init__(self):
        super().__init__()

        loader = QUiLoader()
        loader.setLanguageChangeEnabled(True)
        print(loader.isLanguageChangeEnabled())

        self.app = QApplication.instance()

        self.trans_main = QTranslator()
        self.trans_main.load('ttt.qm')
        self.app.installTranslator(self.trans_main)

        self.main_ui = loader.load('test.ui')
        self.setCentralWidget(self.main_ui)
        self.main_ui.pushButton.clicked.connect(self.trans_ui)
        ......

    ......

    def load_qm(self):
        self.trans_window = QTranslator()
        self.trans_window.load('test.qm')
        self.app.installTranslator(self.trans_window)

    def trans_ui(self):
        self.load_qm()
  
    ......

All text in 'ttt.ts' can be translated at the beginning, but this is not what I want.
Problem 2: what's the difference between this two case? I'm puzzled on what the code is doing.

See https://doc.qt.io/qtforpython-6/tutorials/basictutorial/translations.html
and https://doc.qt.io/qtforpython-6/PySide6/QtUiTools/QUiLoader.html?highlight=quiloader#PySide6.QtUiTools.PySide6.QtUiTools.QUiLoader.setLanguageChangeEnabled

Note we generally recommend using pyside6-uic for Qt Designer forms (see https://doc.qt.io/qtforpython-6/tutorials/basictutorial/uifiles.html ).

I don't do translation, but one problem I see: I believe the original English (or whatever you type into the Designer) gets completely replaced by the translation (e.g. in QUiLoader().load()). So I don't know when you want to re-translate you are supposed to find the original text to lookup from?

You might want to try running pyuic (or whatever) on your project or a sample one just to look at the code it generates for retranslateUi(). You will see it's nothing "magic", but has the advantage if called again that it knows exactly which widgets need translating.

Google also for something like qt translation dynamic to see what work you have to do, e.g. an old link is https://forum.qt.io/topic/55200/dynamic-translation-of-strings-in-widgets.

        self.trans = QTranslator()
        self.trans.load('../../message.qm')
        app = QApplication.instance()
        app.installTranslator(self.trans)

I move the four lines before QUiLoader().load(), and yes, it works. The UI can be translated successfully. But still, there's a problem. If I want to change the language real-timely，is it possble? I mean that in my UI there are several buttons. Each button means a certain language. By clicking these buttons, the UI can directly update the language without restart the APP. If so, seems I must use retranslateUi function. But I can't get it if use QUiLoader().load().

But QUiLoader.load() returns QWidget so should type-casting be there?...

Don't know what you mean here. You are Python, so what "type-casting"??

retranslateUi() is not a method on any widget. It is a little function which is produced by running the "pyuic"-type program for what PySide/PyQt you use on the .ui file produced by the designer. But you don't run these, so I don't think you have one anywhere. See e.g. https://stackoverflow.com/questions/66361580/what-does-retranslateui-do-in-pyqt.

But QUiLoader.load() returns QWidget so should type-casting be there?...