title: java message bundle - property file naming convention when using extensions or private use subtags?
body: <p>What's the naming convention for message bundle property files when using extensions or private use subtags?
  For instance, if the locale is EN US, the property file lookup will be</p>
  <pre><code>message_en_US.properties
  </code></pre>
  <p>though, what if I need instantiate a local EN US and the private use subtag string is <code>RandomTag</code>. How do I name the file so <code>GetBundle</code> can find it? <a href="https://docs.oracle.com/javase/7/docs/api/java/util/ResourceBundle.html#getBundle%28java.lang.String,%20java.util.Locale,%20java.lang.ClassLoader%29" rel="nofollow noreferrer">The API doc does not talk about how to deal with extensions</a></p>
  <p>P.S: I realize <code>RandomTag</code> can be specified as a variant and I could just name the file <code>message_en_US_RandomTag.properties</code> but I would like to avoid this as we might use legitimate variants in the future as specified in the <a href="https://tools.ietf.org/search/bcp47#section-2.2.5" rel="nofollow noreferrer">BCP-47 standards</a> for locale</p>