Add option to set language of openweathermap sensor, and handle updating errors

[azogue]

Description:

Add new config option language to set the language code (two-characters string) in which you want text results to be returned by the PyOWM library.

Also, handle KeyErrors occurring in the library when trying to parse None strings.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2819

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: openweathermap
    name: Local Weather
    api_key: !secret openweathermap_api_key
    forecast: 1
    language: es
    monitored_conditions:
      - weather
      - temperature
      - wind_speed
      - humidity
      - pressure
      - clouds
      - rain
    scan_interval: 180

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

[fabaff]

Looks good to me 🐦