Godot version:
Godot 3.2.3

OS/device including version:
Ubuntu 20.04 LTS

Issue description:
Translations can't be used in editor plugins, making supporting multiple languages difficult.

Steps to reproduce:

1. Add a translation file to the project, either through Project Settings or via TranslationServer.add_translation
2. Attempt to use a translation key
3. See how it doesn't work in the editor, but DOES work when the game is launched.

Minimal reproduction project:
TranslationServerInEditor.zip