RTL language not recognized as such

I have a site in a RTL language. I specified the language in Settings, General of the WordPress dashboard. I tried several themes by different authors that support RTL languages, including twenty-eleven and twenty-ten. Unfortunately, none of them display the site properly. With WordPress 3.4.2, Firebug revealed that the uploaded HTML included the correct language name, but incorrectly said dir=ltr. (I upgraded today to 3.5, and the “dir=” no longer appears.) It seems that the theme’s rtl.css was not used. How does a theme know that this file should be used? Why was my language not recognized as rtl? I could not find any code for a link to rtl.css in my theme. Where should it be and what should it look like?