How can I change Chrome settings to test using navigator.language

Use chrome://settings/languages#lang and (important) make sure that the language you selected is the top choice (the preferred language).

More information: https://alicoding.com/detect-browser-language-preference-in-firefox-and-chrome-using-javascript/

On MacOS, the Chrome UI language (window.navigator.language) cannot be set via chrome://settings/languages or even via the --lang command line argument (like on Windows) or environment variables (like on Linux).

The browser accept languages (window.navigator.languages array, note the final s) can be set via chrome://settings/languages and thus via ChromeDriver options.

Therefore, besides manually changing the system language for the OS user, the only option remaining is the -AppleLanguages flag.

For example, to Spanish:

defaults write com.google.Chrome AppleLanguages '(es, es-ES)'

To verify:

defaults read com.google.Chrome

This is the situation as of Chrome build 58.0.3029.110.

As of Sept 2021, on Mac (M1):

To change window.navigator.language:

1. Settings > Advanced > Language > Add Language
2. then bring this language to the top

To change the language of the user interface:

(thanks to @AdamBittlingmayer above)

defaults write com.google.Chrome AppleLanguages '(es, es-ES)'

To verify:

defaults read com.google.Chrome