Change language. POST or GET

Hello,

first of all, apologies if it is not the correct place to ask for this issue. 

I want to know how is the correct way to change the language of a website:
- a link that invokes a HTTP GET method
- a button that invokes a HTTP POST method

AFAIK:

- GET should not be used for operations that cause side-effects. 
- The GET method means retrieve whatever information

So, how can we consider change the language of a website?

What do you think?

Best regards,

Carlos Tejo