::I create a large number of categories, frequently to reduce the size of larger categories or to expand existing series.  Hence the Russian one. 
 ::#For me, machine building is what it say, the building of machines.  If you want to expand that back to the beginning of the process, then it would include the actual designing.  At that point mechanical engineering and electrical engineering would be two of many disciplines that would be involved.  Others could include designers to advise on the appearance of the machines. 
 ::#Any kind of company that provides engineering services.  This could include any of the disciplines.  So you run from airplane design, to power plant design to bridges to bio medical devices. So it is broad and covers a very wide area.   
 ::#A manufacturing company is one that makes anything from a car to a screw.  Slabs of granite for your kitchen counter to the pepper grinder that you put on top of the counter.  Packing plans, those that put oranges in cases for shipping, including the prep, are not manufacturing plants. 
 ::#Engineering companies are not manufacturing companies.  Though a company can employ both types of skill and do both things. 
 ::Now, I'm not a Russian expert, but I think that they have had a tendency to label and do things differently then many other nations.  So what they call something may not jive with the definition used in the rest of the world.  In that case, you have a problem.  Subcategories need to roll up correctly and simply saying something is X may be OK for local usage but that does not mean it rolls up in to Category:X if that is used to collect different items. 
 ::I see you may well need to decide where to change the name of the category and what is included in it.  This will not be trivial and will likely generate some discussion.  Without doing any digging to see what you have been working on, I'd suggest that you bring a trial rename for something in this area to a full CfD.  Explain the problem and offer a suggested rename.  Then see what the others say.