How can I modify a translation in colormag-fr_FR.po? In fact the translation of "breaking news" is "Récents" in french. I went in the above mentionned file and changed "Récent" for "A la Une". But after reloading my website, nothing had changed. I want to be able to modify those translations to put mine. Thanks in advance.
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.