The value stored in general.useragent.locale is only used for the
Firefox user interface (menus, dialogs, etc). Its not sent to servers.
So given Mayuresh's original use case, use intl.accept_languages.
Given you only want to choose between 2 languages, just see which
comes first: "fr" or "nl". Note that its possible for people to have,
for example, "en-US" but not "en" in this list.
- Blair