how to translate MTB wheel specifications?
Rear wheels have never had 9mm axles, they have always been 10mm. For all but "downhill" MTB[2] and fatbikes[3], they are  either 135mm 10mm or the new 12mm 142mm maxle standard. 

http://www.pinkbike.com/news/12x142-explained.html

Unless it specifically states otherwise I would assume that the rear wheel of an MTB wheelset is a 135mm 10mm axle. Anything that "looks" like a standard QR should be 135mm 10mm. But the vendor should make that pretty obvious, and if they don't it should be easy to ask. Wheelsets are expensive, it should be simple to get the answer to what dimensions are used in the rear hub. 

[2]- Downhill MTB has a 150mm thru-axle standard. 

[3]- Fatbikes use asymmetric 135mm axles (hub shifted to the driveside, relative to the rim) 170mm symmetric axles and 190mm/197mm thru-axles