Detect language by using navigator.language

The `window.navigator.language` returns a locale language (e.g. `en-US`) on most browsers.

Also, the `window.navigator.userLanguage` returns the similar data on IE (<= 10), but this data actually returns lower case strings such as `en-us`. In this example, locale string fallbacks to first string (e.g. `en`) from split data.

Just a plan.