I know this isn’t the best solution, but you can still use CSS.
You only need to add a rule for each language, using the correct selector:
/* Instead of using this selector ... */
nav.post-controls .actions button.reply pan.d-button-label:after {
}
/* ... start the selector with the language */
html[lang=pt-BR] nav.post-controls .actions button.reply pan.d-button-label:after {
content: " text in Portuguese";
}