On Site SEO - 1000's of indexed and redirected pages in another language

This is quite a common hack I am afraid. I would imagine some plugin has not been kept up to date which allowed them in. 

I doubt you will find the pages as such as they are generated dynamically, Its a bit hard to say which file they would have re-written to produce these pages but I believe functions.php is a common target. 

Seeing as the site is small it may be the easiest to take a backup of the pages and graphics you want to keep and re-install Wordpress then restore your pages. 

There are quite a few plugins that would prevent this from happening in the future but I am afraid I don't know which is currently the best but a little research should find the most popular if you search for security.

Sorry, I can't help further as I am not a Wordpress user/fan but hopefully you can get it sorted. 

As Chedders has pointed out, this is a very common problem.

If you would provide me your website domain name I will take a look at the site and give you some recommendations.

You can post the domain here in the public or PM me direct.