I think there is a bug in PyQt5 when handling QDialogButtonBox.Open in QFileDialog. If I change the button text, when I select a file in the list it recovers the original text.
With QDialogButtonBox.Cancel there is no problem.
Here is my code:

import sys
import os

from PyQt5.QtWidgets import (QFileDialog, QDialogButtonBox, QApplication, QListView)
from PyQt5.QtCore import (QCoreApplication, QMetaObject)

class OpenFileDialog(QFileDialog):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # Path
        self.path = os.getcwd()

        options = self.Options()
        options = self.DontUseNativeDialog
        self.setOptions(options)

        # Ok and Cancel buttons
        d_button = self.findChild(QDialogButtonBox, "buttonBox")
        d_button.setFixedWidth(110)
        self.but_ok = d_button.button(QDialogButtonBox.Open)
        self.but_ok.setText("Seleccionar")
        self.but_cancel = d_button.button(QDialogButtonBox.Cancel)
        self.but_cancel.setText("Cancelar")

        self.but_ok.clicked.connect(self.click_ok_button)
        self.but_cancel.clicked.connect(self.click_cancel_button)

        # My solution
        fil_lst = self.findChild(QListView, "listView")
        fil_lst.clicked.connect((self.selectitem))

        # Call translate
        self.retranslate(self)

    def selectitem(self):
        self.but_ok.setText("Seleccionar")

    def click_ok_button(self):
        print("Select file")

    def click_cancel_button(self):
        self.close()

    def closeEvent(self, event):
        print("Select None")

    def retranslate(self, open_file_dialog):
        _translate = QCoreApplication.translate
        self.but_ok.setText(_translate("Open", "Seleccionar"))
        self.but_cancel.setText(_translate("Cancel", "Cancelar"))

if __name__ == '__main__':
    app = QApplication(sys.argv)
    dialog = OpenFileDialog()
    dialog.show()
    sys.exit(app.exec())

I tried changing the text with Qt translator and the problem happens too.
I solved it by changing the text every time the list is clicked, but it is not a pretty solution.
Does anyone know a more pythonic solution?