From JC2-MP Documentation
Does my version of the game work with JC2-MP?
The retail boxed version and the Steam version are the same. You should have 188.8.131.52 by default, as Steam maintains this version. This is the only version the mod supports. Moreover, the game tests and development are conducted on the English version of the game; your mileage may vary with other languages.
Non-Steam installs or cracked installs will NOT be supported.
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