Add language option to config

[JonnyWong16]

Description

Adds the language option to the config which is passed as the X-Plex-Language header. The language is an ISO 639-1 language code. The default is en.

Closes #1234

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable