Just modifying the .po file doesn’t do anything. The .po is the template. To be displayed, the .po must be compiled into an .mo file and it is the compiled .mo that makes the translations.
If you’re just editing the .po in a text editor, you’re not changing anything. You’ll need a compiler.
A good choice is Poedit. It’s been around a long time and is used by a lot of WordPress developers. It will allow you to edit the .po file and it will automatically compile the .mo when you save it.