Support for config option of changing default transcription language in jigasi

By default there seems to no way to change default language model used by Google speech to text (which is currently set to en-US), Propose to add a way to change a way to set default language model through config file sip-communicator.properties
Request developers (@damencho) to please take a look pull request https://github.com/jitsi/jigasi/pull/304 and see whether this will work or suggest better way to change this setting.