Themes and language packs are two different thing.
Themes - A theme is a collection of property settings that allow you to define the look of pages and controls, and then apply the look consistently across pages in a Web application, across an entire Web application, or across all Web applications on a server.
Language packs - Language packs are used to enable support for specific languages and locales.
If you have downloaded / installed a 3rd party theme, you need to download / install your language pack separately.
If your site's primary language is Right-to-left (RLT) then you need to see if the 3rd party theme supports RLT or not.