Hi! Thank you for your answer.
I still wondering, is there a function I can add in my theme for language switching option?
If I use the plugin, I need to redesign the classes and if the plugin will be updated (and classes are changed), I have to redesign all it again and again.
I do not want to use machine translation. I do not need to translate pages or posts content, because there are mainly pictures, galleries, videos and design-portfolio.
But it will be nice, if the user can choose either, read the site (and also the menus, headings, titles, categories and tags) in English or in my local language.
What is the simplest way to do that for my theme (only my own use)?