Return fullpath when key not found

[mweibel]

When using a key which is not found, IMHO we should return the full path instead of just the last part of the iteration. This makes it possible to use a fallback if the passed key is the same as the returned key.

Though: I guess you had a reason why you returned just the last key. Should we maybe instead add an optional fallback argument?

[alexkuz]

It's just so the page wouldn't seem messy if langpack value is not defined yet (or langpack was not loaded or something).
I think it's better to have an optional notFoundCallback, by default - (locale, path) => path[path.length - 1]. I guess i18n should be a factory that receives options argument:

const DEFAULT_OPTIONS = {
  notFoundCallback: ...
};

export default function i18n(options) {
  options = { ...DEFAULT_OPTIONS, ...options };

  return (state = DEFAULT_STATE, action) => {
    return (action.type === LOAD_LANG_SUCCESS) ?
      mergeLang(state, action, options) :
      state;
  }
}

and then pass it to i18nResolver, something like that.