What? The Wii uses the GX API from the GameCube [0], which is not similar at all to GL 3.3; it was developed in 1999. It does not have any form of programmable shaders; it uses TexEnv-style combinators.
The Wii U has a custom GX2 API, where the official shader language is GLSL (again, see the Mario Kart 8 kiosk demo).
The Switch has NVN, GL, Vulkan, where the official shader language for all three is GLSL (based on NVIDIA's GLSL compiler).
Well, first of all, when I wrote Wii, I wasn't even thinking into the distinction between Wii and Wii U.
Secondly, the Cafe SDK documentation clearly refers to OpenGL 3.3, and GX2 is basically OpenGL in spirit while using naming and some utility functions, which fits "GL based API similar to GL 3.3".
I assume you also have access to Cafe documentation.
The Wii U has a custom GX2 API, where the official shader language is GLSL (again, see the Mario Kart 8 kiosk demo).
The Switch has NVN, GL, Vulkan, where the official shader language for all three is GLSL (based on NVIDIA's GLSL compiler).
[0] https://libogc.devkitpro.org/gx_8h.html