For Nightwatch, what I did was specify my driver to launch in the specific language/locale settings in the Nightwatch.conf.js file and then launch the test with the --env setting corresponding to that language.
I then leveraged a framework called i18next which let me use lookup keys in place of hardcoded strings in my tests so I didn't have to create multiple test files for each language. The test automatically detects which language the browser context is in and looks up the correct string values.