I guessing it's Chromium's default language. Whenever I install Firefox or Chromium/Chrome the browser settings are always en-US and I have to update it in preferences.
Perhaps there's a way you could update it in Electron's APIs. If you can't do this and it pertains to web requests then you may be able to intercept outgoing HTTP requests and edit the language header before it is sent.
For my app I looked in the folder where Electron's Chromium stores its data and there is a
Preferences file but it seems to be drastically reduced compared to a normal browser install, as it only contains settings relating to the Dev Tools window when I last used it.