Yes for this u would need to Compile the Client source with LC_FRA = X? (example LC_USA = 7 in client i think, in server its 9)
U would need to find the same Client version from the fra client and patch the Engine.dll to read the sl.dta if you find me the client and upload it on mega i can try it out to patch it i had a tool it.
Edit:
Or u dont even have to do that, just Compile the Server with LC_FRA settings and use the french client just find the correct version.
I dont know now im not home to lookup in the source
you need to go to your Project Settings -> C++ -> Preprocessor and change the define from LC_KOR = 0 to LC_FRA = ??
(if you compile it on Linux its more simple)
this is how the Preprocessor looks idk if you have knowledge in coding but if you had u wouldnt ask the question.
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EDIT one again:
its not the nksp that choose the names from the .lod files its the engine.dll the function is/was in World or WorldIO.cpp i dont know exacly now.