There is only one file type that uses the .MSTXT extension:
Message Studio Text File
An MSTXT file is a text document used to create dialog translations for a Nintendo video game. It contains in-game dialog written in a variety of languages, each of which is organized into a separate XML category. MSTXT files are used to create .MSBT files.
Nintendo's internal translation team uses a program called Message Studio to generate in-game dialog and other text for Nintendo video games. The text files that Nintendo's translation team edits within Message Studio are MSTXT files.
Initially, MSTXT files contain only the Japanese dialog used in the Japanese version of a Nintendo game. As Nintendo's translation team localizes the game for additional regions, they add translated dialog written in the game's other languages. When the translation team has finished adding translations for a game, they save each language's in-game dialog within its own, individual MSBT file. These MSBT files are then included in each region's version of the game.
MSTXT files are XML files, so they can be opened in any XML editor, such as SyncRO Soft oXygen XML Editor (Windows), Bare Bones BBEdit (Mac), or SCREEM (Linux). They can also be opened in any text editor.
Updated: August 28, 2020