Buttons from open/save dialogs are not displayed as translated

[Rafael Fontenelle <rafaelff gnome org>]

An user notice that some buttons from some non-GNOME software with
label "Open", "Save" and "Cancel" are not being displayed as
translated. This affects GTK implementation of the Open and Save file
dialogs. Software listed so far are Chromium, Opera, Vivaldi and
l3afpad.  I double-checked: these strings (Open, Save, Cancel) are
translated in all the software (I can't tell for Opera and Vivaldi,
because it is not open source), so should not be displayed in original
language. It was reported to GNOME 3.26, but I reproduced it in 3.24
too.

Any idea what could be causing this, or where should I be looking at?

See images and more in the bug report:
https://bugzilla.gnome.org/show_bug.cgi?id=786864

Best regards,
Rafael Fontenelle