How to create
A language file can be generated by CONFIG.COM, which can be found on the internal DOSBox Z: drive when you start up DOSBox.
CONFIG -writelang filelocation
It writes the current language settings to a file in a specified location. "filelocation" is located on the local drive, not a mounted drive in DOSBox. The language file controls all visible output of the internal commands and the internal DOS.
How to change
Read the language file that you created from the instruction above, and you will hopefully understand how to change it.
How to use in DOSBox
You can setup the filename in the configuration file (DOSBox.Conf) in the [dosbox] section. There's a language= entry that can be changed with the filelocation.
Alternatively, start DOSBox with the -lang switch to use your new language file.