With the first start of LibreCAD you should have been asked for the language and drawing units. I have no clue how this can happen by accident. The default language is English and the units mm.
You can change these settings in Application Preferences. But this can be tricky when you don't know the current GUI language.
Another option is to modify the preferences in the registry or config file. It depends on your OS where you can find the preferences this way.