The [ISO 639-1](https://en.wikipedia.org/wiki/ISO_639-1) language code, followed by an underscore
and the [ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2) country code
For example: `es_MX` - "Spanish (Mexico)".
If supplied, the returned content will be in the specified language.
If the locale isn't supplied or the specified language isn't available, all strings will be returned
in Spotify's default language (American English)