https://stackoverflow.com/questions/54976933

lang_es.ts

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="es">
<context>
    <name>Form</name>
    <message>
        <location filename="main_ui.py" line="31"/>
        <source>Form</source>
        <translation>Formulario</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="32"/>
        <source>save</source>
        <translation>guardar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="33"/>
        <source>cancel</source>
        <translation>cancelar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="34"/>
        <source>send</source>
        <translation>enviar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="35"/>
        <source>EN</source>
        <translation>EN</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="36"/>
        <source>PT</source>
        <translation>PT</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="37"/>
        <source>ES</source>
        <translation>ES</translation>
    </message>
</context>
</TS>

lang_pt.ts

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="pt">
<context>
    <name>Form</name>
    <message>
        <location filename="main_ui.py" line="31"/>
        <source>Form</source>
        <translation>Formulário</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="32"/>
        <source>save</source>
        <translation>salvar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="33"/>
        <source>cancel</source>
        <translation>cancelar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="34"/>
        <source>send</source>
        <translation>enviar</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="35"/>
        <source>EN</source>
        <translation>EN</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="36"/>
        <source>PT</source>
        <translation>PT</translation>
    </message>
    <message>
        <location filename="main_ui.py" line="37"/>
        <source>ES</source>
        <translation>ES</translation>
    </message>
</context>
</TS>

main.py

import sys
from PyQt5 import QtCore, QtWidgets

import main_ui

class ExampleApp(QtWidgets.QMainWindow, main_ui.Ui_Form):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        self.en_btn.clicked.connect(self.retr)
        self.pt_btn.clicked.connect(self.retr)
        self.es_btn.clicked.connect(self.retr)

        self.trans = QtCore.QTranslator(self)

    @QtCore.pyqtSlot()
    def retr(self):
        text = self.sender().text()
        d = { 
            "ES" : "lang_es",
            "PT": "lang_pt"
        }
        data = d.get(text)
        if data:
            self.trans.load(data)
            QtWidgets.QApplication.instance().installTranslator(self.trans)
        else:
            QtWidgets.QApplication.instance().removeTranslator(self.trans)

    def changeEvent(self, event):
        if event.type() == QtCore.QEvent.LanguageChange:
            self.retranslateUi(self)
        super(ExampleApp, self).changeEvent(event)

def main():
    app = QtWidgets.QApplication(sys.argv)
    window = ExampleApp()
    window.show()
    app.exec_()

if __name__ == '__main__':  
    main()

main_ui.py

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(364, 135)
        self.save_btn = QtWidgets.QPushButton(Form)
        self.save_btn.setGeometry(QtCore.QRect(30, 30, 85, 27))
        self.save_btn.setObjectName("save_btn")
        self.cancel_btn = QtWidgets.QPushButton(Form)
        self.cancel_btn.setGeometry(QtCore.QRect(150, 30, 85, 27))
        self.cancel_btn.setObjectName("cancel_btn")
        self.send_btn = QtWidgets.QPushButton(Form)
        self.send_btn.setGeometry(QtCore.QRect(260, 30, 85, 27))
        self.send_btn.setObjectName("send_btn")
        self.en_btn = QtWidgets.QPushButton(Form)
        self.en_btn.setGeometry(QtCore.QRect(30, 80, 85, 27))
        self.en_btn.setObjectName("en_btn")
        self.pt_btn = QtWidgets.QPushButton(Form)
        self.pt_btn.setGeometry(QtCore.QRect(260, 80, 85, 27))
        self.pt_btn.setObjectName("pt_btn")
        self.es_btn = QtWidgets.QPushButton(Form)
        self.es_btn.setGeometry(QtCore.QRect(150, 80, 85, 27))
        self.es_btn.setObjectName("es_btn")

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.save_btn.setText(_translate("Form", "save"))
        self.cancel_btn.setText(_translate("Form", "cancel"))
        self.send_btn.setText(_translate("Form", "send"))
        self.en_btn.setText(_translate("Form", "EN"))
        self.pt_btn.setText(_translate("Form", "PT"))
        self.es_btn.setText(_translate("Form", "ES"))

Steps:

1. Execute pylupdate5:

    pylupdate5 main_ui.py -ts lang_en.ts
    pylupdate5 main_ui.py -ts lang_pt.ts
   

2. Open *.ts files with Qt Linguist and edit the necessary fields:

3. Execute lrelease:

    lrelease lang_es.ts lang_pt.ts
   

4. Modify the main_ui.py so that the translation is loaded according to the button pressed using the load() method of QTranslator, this change will cause the GUI to send the event QEvent::LanguageChange to changeEvent() so that method is overwritten, and in that method it has to be call retranslateUi() method.

Results: