Website locale design: using cookie or not

For a website available in multiple locale, each locale specific page has a different URL. For example if i go to gmail and switch to french, now the URL has "intl/fr/" inside it, which is used to determine the locale. If i change the URL manually to gmail.com, it goes back to the default English version again. This looks like correct behavior to me.

But if i do the same on slack.com the results are different. Slack sets the locale in a cookie, and that determines the locale, rather than the URL. So once i choose french, typing slack.com in the browser will show it in french always (and same for all pages)

Which of these do you think is a better design or more intuitive behavior?