Hello!
After all night testing Im sure problem is related to ngnix server configuration.
I have nginx server, configuration should be ok (other cms works).
rewrite directive is correct:
location / {
        try_files $uri $uri/ /index.php?$args;
    }

problem:
If I swich language (I have 2 languages) when Im on categories appears error.
for example:

https://mysite.com/category-name-in-language-1
now I wanna switch this page to second language, so content should change and url change to
https://mysite.com/category-name-in-language-2

but what happens:
after switching to second language url doesnt change and in content appears:
" Whoops, something is wrongWe are sorry, but the Web address you have entered is no longer available.".

If I go to listing:
https://mysite.com/category-name-in-language-1/listing_in_language_1 and change to second language everything is ok, url changes to:
https://mysite.com/category-name-in-language-2/listing_in_language_2


so the problem appears only in categories.
I tested on new installations, etc. with and without listing, with subcategories - all the same.

I googled trying to find solution - nothing (rewrite directive above).

Maybe someone have nginx and can help me???

there can be confusing situations for users and I think this error can destroy seo.


I was so long searching for solution, that I dont believe, I will resolve this probl.

Maybe, there is another solution:
most people, logically, choose language on homepage (because this is main and first page), is there possibility show language block only on home page?

@calm
To show block for languages just on homepage, you can wrap it with:

<?php if(osc_is_home_page()) { ?>
  ... block ...
<?php } ?>

thanks for replay @frosticek!

I figured this too by myself ((so much hours...)) if I I would know, that You will replay, eh)!!

@calm
Welcome