Custom package with description.lua and just the necessary textures taken from Quatermaster's "MiG-21bis Complete Metric English Cockpit - 1.5.4 & 2.0.2+" texture replacement for the Fishbed.
Tested on 1.5.6 and 2.0.5, should be stable as long as this feature is not changed by Leatherneck Simulations.
See forums: https://forums.eagle.ru/showthread.php?p=2980942
This uses the "custom" cockpit feature of the Mig-21Bis and offers a choice instead of replacing the whole cockpit textures. It is also much lighter since it only replaces the necessary textures to get a fully translated cockpit - at the exception of the stick ;-)
The description.lua file in Mods\aircraft\MIG-21bis\Liveries\Mig-21_CP\custom is not removed anymore, since update 184.108.40.20690 and 220.127.116.1189. So normally, once it is installed, it will remain available through the DCS updates.
Probably not supported correctly by 2.1 and later.