Check the language code command

[mattberjon]

In order to detects error in the selection of the language code selection, it's necessary to check the consistency of the argument.

[mattberjon]

I didn't come up yet with handy and nice solution to ensure the consistency of the language written. It seems, that if the API get a wrong language code (that doesn't exist in the list or doesn't respect the IETF BCP47 recommendation, then by default, the american language detection will be used (en-US code).

Several possibilities can be explored (but maybe not always possible):

• Find a way to asynchronously get the language used by the speech API and return a message to user to inform him which language is currently activated (not sure if the API has this method).
• Use a file containing all the current available codes by the speech API and check the sanity of the argument entered by the user against this list. This can be a bit improved by using a dedicated language code library in order to help a bit the user by using some best_match() method if he make an easy syntax error.