I think localisation would fit better in the main project workspace, especially when you’re switching between editing translations and designing your interface.
My proposal is to add a Localisation item beneath each project in the navigator. Clicking it would open a dedicated editor tab alongside your code and window designers. You could leave it open and switch back to it without reopening Project Settings or resizing a separate window.
The editor would keep the translation table, locale controls, validation, and JSON import/export. Changes would use the normal project saving and undo/redo behaviour. The existing localisation entry in Project Settings could become a shortcut to the same editor.
I’d initially keep this as one Localisation item per project, with the languages together in the editor so you can compare translations side by side.
Would this suit how you work? In particular, would you prefer that single editor showing languages together, or would separate language entries in the project tree be more useful?