Okay, so this is a little odd. This does not effect the language or direction of the website, but instead is a measure to ensure proper encoding of the CSS file itself.
The browser will generally rely on the HTTP Headers to determine this value, but in cases where the server or application configuration does not, you can provide the equivalent in the file itself.
WARNING: This needs to be the first line of the .css file, before any spaces or comments.
Other common value: