Subtitles will be searched for in a priority order (click on an image to zoom).
So be sure to enter your native language in the first box. Then add all other languages that you can understand, from good to bad.
Encoding: Encoding of the characters in the downloaded files. For Europe and USA 'Auto Detect' is usually the right thing to enter. Note that for non-western languages the corresponding character encoding must be manually selected. 'Auto Detect' doesn't work here because eastern and asian languages reuse some characters differently than western languages. Just keep this in mind when you download subtitles for eastern or asian languages and you see scrambled characters after the download.
List of Codes: Comma separated list of abbreviations (codes) for the selected languages. This is the technical format of the four dropdown boxes above. You can use the copy button on the right and carry it over to some video player settings as for example VLC in the Preferences dialog in the section "Input/Codecs->Subtitle language" or in older versions in the section "Subtitles". You can enter something in the box which will alter the dropdown boxes.
The language list of about 70 languages is downloaded from the OpenSubtitles.org server. This means it can expand in the future. Click the update icon to refresh the list from the server if you are missing a language.