If I'm not mistaken, Wii games at least have a common file system, and the first step would be to extract files with a utility such as Wimms ISO Tools, available at https://wit.wiimm.de/
But beyond that, there is no such thing as a universal tool to "extract and repack text data" as different games frequently store "text data" in completely different ways. To determine how any individual game stores its "text data" is a frequently arduous process requiring extensive trial and error, even if you "don't want to do anything extreme". (Sometimes things that look like text are actually stored as images, and then things can get even more complicated.)
Why not say what specific game you want to change? Maybe someone has done some work on it already.