At my website I have a button when pressed PHP script through ajax write session variable to change language and reload page. But when I enable caching every time is loading default language. When cache is disabled everything is OK.
How translations do big pages with caching?
Topic translation plugin-w3-total-cache php cache Wordpress
Category Web
If you have any kind of caching enabled on a site with Wordpress, IMHO, it's probably best to use cookies instead of session variables, because with caching enabled, the user's request for a page may not even hit the server at all — so the session var isn't even being read.
What I typically do is use cookies — both setting AND reading them on the front end via ajax. You could dynamically load the new language content that way.
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Adding on to answer your second question. If you look at a plugin like WPML or something similar, they are actually creating duplicate content in many cases. So a site might be mydomain.com/en for English and mydomain.com/es for Spanish. Each endpoint is cached separately. Some cookies are probably set as well, but when you switch languages with WMPL, you are actually redirected to a different page, not the same page with different language content... if that makes sense?
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