getBestMatchingLanguage should return null as a last resort

[1ec5]

getBestMatchingLanguage() currently returns en for English if all the other fallbacks have been exhausted. Instead, it should return null as a last resort.

osrm-text-instructions/index.js

Line 295 in ed70d2a

return 'en';

The current behavior is problematic, because the caller may not know whether this method returned en because it’s the best match or because there’s no other option. A client may want to use this method with a list of acceptable languages, such as navigator.languages, stopping only when a match is found. With the current behavior, the client would have to either ignore every language after the first one (because this method always returns a value) or ignore every language other than English (because this method uses English as a last resort).

/cc @mcwhittemore @lyzidiamond @allierowan @bsudekum