The script determines from which page the link was clicked and redirects it back to that page after the theme change.

1) First, match the theme - language in the admin panel (for the language to change when the theme is changed. In the Templates section)
2) Put the change.php file in your site's root directory.
3) If you want to specify a short link for theme change, add it to your .htaccess file as follows. (Under RewriteEngine Ten)

RewriteRule ^en$ change.php?skin=Gemini [L]
RewriteRule ^tr$ change.php?skin=Default [L]

RewriteRule ^mobil$ change.php?do=mobile [L]
RewriteRule ^pc$ change.php?do=pc [L]

Gemini and Default theme names are specified in the codes in the first two lines.
Once your website.com/en is written, the Gemini theme will be active.
When you type your website.com/fr, the Default theme will be active.

Your website.com/mobil Mobile theme will be active when it is written. ( If active )
When you type your site.com/pc, the Normal theme will be active.

If you don't want to use a shortcode like this. You need to use links like the example below
website.com/change.php?skin=Gemini

If you want to make the change with select without a link, edit it by adding the following code to a suitable place in your theme.

<form method="post" action="/change.php">
    <select onchange="submit()" name="skin">
        <option value="Gemini">Turkish</option>
        <option value="Default">English</option>
        <option value="Pisces">Russian</option>
    </select>
</form>

Gemini => Turkish
Default => English
Pisces => Russian