How to use different designs for different languages?

For language dependent CSS rules, you can use the :lang CSS selector.
Languages written from right to left use the rtl.css stylesheet already included in most themes.
It』s also possible to apply a different stylesheet for each language. Indeed, stylesheets named liked en_US.css, fr_FR.css or ar.css put in your theme directory will be automatically loaded by WordPress.
Note that in every cases, you have to keep the main style.css file.