This is an editor plugin to edit translation files from inside Godot Engine. It supports .csv and .po files, although gettext support is limited to what Godot can handle. It can also be used as a standalone editor if translation_editor.tscn is run or exported as main scene.
It can extract strings from your project automatically and adds them to a list for you to translate. There is no particular convention to use (GDScript and TSCN files have special parsing), but you can specify a prefix, in which case any common text format should work.
This is a regular editor plugin. Copy the contents of addons/zylann.translation_editor into the same folder in your project, and activate it in your project settings.