Hi maxolino,
If you are creating translation file then you can add your translation file .po and .mo file inside the language directory. I think you have made a direct changes in the theme. Doing this while updating to new version all your customized code will be lost. So better make a child theme so that your code customized code will not be lost.
The previous/ next text is inside theme folder-> inc -> structure-> content-extension.php and search your term.
Thank you
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