[Python] Add convertLanguage(const char* language, int format) method.

This PR allows addon devs to convert one language format to another.
The method works as follows:
```
    /**
    * convertLanguage(language, format) -- Returns the given language converted to the given format as a string.
    *
    * language: string either as name in English, two letter code (ISO 639-1), or three letter code (ISO 639-2/T(B)
    *
    * format: format of the returned language string
    *         xbmc.ISO_639_1: two letter code as defined in ISO 639-1
    *         xbmc.ISO_639_2: three letter code as defined in ISO 639-2/T or ISO 639-2/B
    *         xbmc.ENGLISH_NAME: full language name in English (default)
    *
    * example:
    *   - language = xbmc.convertLanguage(English, xbmc.ISO_639_2)
    */
    String convertLanguage(const char* language, int format); 
```