James Lang was a relative of Joseph Lang (brother?) that also made guns. James Lang is not the same as saying "I have a Lang per se". James Lang's guns are of a lower quality than Joseph Lang's arms and they were not produced for as long a period of time. (e.g. Joseph Lang brought the breach loader to the British market, his firm invented the 470NE, his successors are still in business today as Atkin, Grant & Lang)
As to quality and value, the shotgun appears to be a base model 12 bore hammergun. Its black powder proofed with Damascus barrels and of course 2.5" chambers. It's overall condition is poor.
Value depending on wall thickness, chokes, and a closer inspection would be $500-$1200 if it is not broken and not pitted. It could very well be worthless as well if off face, heavily pitted, or if the walls have been honed down to conceal wear.