you missed xcc mixer, which allows importing and exporting from mix files. somewhere on ppm forums.
But openRA does not rely on mix files. you can add any
new content without mix files.
The prime reason why it is not possible to translate is that "THERE CAN ONLY BE ONE!" language at the same time in OpenRA code at the moment. (excuse the joke) Why? Because the language lines are "hardcoded" atm (not really but hardcoded enough). The devs would fist have to put switches or abstracted lists everywhere which are easily/dynamically switchable. It's not done yet!
General note: As this is a hobby project for all developers it is frowned upon to demand stuff from the devs. That certainly gets you nowhere.
The documentation may be a bit lackluster for the c# code stuff, but I think the general idea is that IF you can code with c# and MSVS2017, then you'll find your way around. There are modern software development methods which acutally
encourage low/no documentation, since, as you noticed, documentation quickly outdates.
Also, Ronald, you are learning everything at once, on every level. Mapping, modding,(<-there is a difference here->) coding, modeling, software development etc. A truly monumental effort. Please gather some more experience before blaming others for providing not enough supportive material for you. There should be tutorials for everyhting on the internet.
On you blender question, idk where you posted that - in blender you can choose "isometric" camera, then make an animation which rotates the camera in 8 or 16 or 32 steps around the model you made and have it save as seperate images. then you have a sequence of images which shows your model in 8/16/32 facings. Use SHP builder to import those images (you have to rename first to 001..032) and save as shp(ts) file (even if you are going for RA models). Put that .shp file in a custom map and edit the rules file of that map. You must modify the sequences: section (not rules: (!)) and re-add the complete sequence definition of unit from the mods/ra/sequences folder. Exchange or add "mytupolevbomber.shp" behind the definition for the "mig: <here> idle:" for example. Then your tupolev model should be in the game on this one map.
As you can see it is a complex process.
Happy modding
Cheers