Kuriimu

A powerful and flexible feature-complete editor for several text formats from a wide range of games, including MSBT, the text format used by NSMBU/NSLU to store 99% of the game's text.

IMPORTANT: You may see big warning messages on Kuriimu's websites such as the GBATemp thread and the GitHub telling you to move to Kuriimu2, a new successor to the original Kuriimu (this one), however the new Kuriimu2 DOES NOT YET SUPPORT MSBT, given it's still in heavy development, so DO NOT MOVE TO IT YET, download the original version linked here on this page, it still works fine.