from PySide6 import QtCore
app = QtCore.QCoreApplication([])
class Probe(QtCore.QTranslator):
    def isEmpty(self): return False
    def translate(self, context, source, disambiguation=None, n=-1):
        if source == 'known': return 'известно'
        return None
translator = Probe(app)
print('installed', app.installTranslator(translator))
print('known', app.translate('test', 'known'))
print('unknown', app.translate('test', 'unknown %1'))
print('base', QtCore.QTranslator().translate('test', 'unknown'))