I am trying to switch themes when the site language is switched. Is it possible to have separate themes for each language in a Drupal 7 multilingual site?
Yes - you can create different themes for just about any condition you like as long as you have a way of switching between them. There are several theme switcher modules available but one of the better ones is 'ThemeKey' which does what you are looking for:
ThemeKey allows you to define simple or sophisticated theme-switching rules which allow automatic selection of a theme depending on current path, taxonomy terms, language, node-type, and many, many other properties.