MATL, 18 bytes

The current version (4.0.0) of the language is used.

This applies the same regular expression as NinjaBearMonkey's answer:

j'^[A-Z][a-z]+$'XX

The output is the string (which is truthy) if it's an official name, and nothing (which is falsy) if it's not.

###Examples

>> matl
 > j'^[A-Z][a-z]+$'XX
 > 
> December
December
>> 

>> matl
 > j'^[A-Z][a-z]+$'XX
 > 
> ASCII
>>