Whats the difference between available region during Nintendo Switch setup, and is my choice irreversible?

Solution 1:

The region affects visual changes in some games and the choice is reversible as many times as you want.

You can change the region and language settings as many times as you want. The location of this setting is...

HOME Menu > System Settings (Gear Icon) > System > Region
* Note that this requires a restart of the Switch.

This can affect aesthetic changes in many games if they are set up to take data from the console. For example in The Legend of Zelda: Breath of the Wild the temperature could be listed as either oF (when set to the US) or oC (when set to Australia). Changing this may also affect the language choices available.

As a personal anecdote; I have bought a Nintendo Switch in Japan with a Japanese The Legend of Zelda: Breath of the Wild game (physical copy). Changing the locale settings to Australia and the language to English has made the games text, dialogue and startup images change from Japanese to English and the displayed temperature appear as Celsius.