You can do this using two calibre configurations, where each configuration specifies a different language and library. You can specify a configuration by setting an environment variable before starting calibre. See "calibre-portable.bat" in the resources folder for full documentation.
I have 5 different configurations (my production library and 4 for debug & development). Each one has an associated .bat file. For example, my production version prod.bat file contains
set CALIBRE_CONFIG_DIRECTORY=C:\CBH_Data\My Dropbox\Charles\config_production
The .bat files use calibre-debug -g because I always want to see debug output. You might choose to use "calibre".