Sorry for our late reply.
For the first question:
The top language switcher is just html layout.
To make the real language switcher, you need to install WPML plugin.
In the WPML, if you set your multi-language, you can see the languages which you set on the language dropdown.
You can get the WPML doc from next link.
If you needn't set the multi-language website, please install loco translate plugin, and translate your website to your own language.
For the second problem:
Please go to admin->theme options->single product->filter tabs, and then add the additional tab content like screenshot.
You can add your html block shortcode in that field.
If you want to add the special tab content for the special product, please go to single product admin page, and add the additional tab title & content like screenshot.
For the last question:
You mean you want to change the "menu" & "cateogries" text in there?
Those texts are all static texts which is wrote on php code. So you can translate it to your language but didn't change the words.
If you have any other questions, please let us know again without hesitation.
Best Regards, C-Themes