Currently language is only supported by the default TMDb and TVDb providers. You can use the --language setting to set the language used for templating. This can be set using either a ISO 639 language code or name. For instance, if I wanted to rename a file using russian I could use rus, ru, or russian. Below is a listing of supported options. It is based off the most frequently used language codes that have provider support and is not meant to be comprehensive.

Protip: If you are using a configuration files and subtitles are being skipped over make sure that ".srt" is included under "mask".