Update MDN/Contribute/Localize (#3123)

Update the discussion place of Japanese translation community
- Google Group is less used
- Slack and GitHub is actively used

files/en-us/mdn/contribute/localize/index.html

@@ -37,7 +37,7 @@ <h3 id="French_fr">French (fr)</h3>
 <h3 id="Japanese_ja">Japanese (ja)</h3>
 
 <ul>
- <li>Discussions: <a href="https://groups.google.com/forum/#!forum/mozilla-translations-ja">Mozilla.translations.ja</a></li>
+ <li>Discussions: <a href="https://mozillajp.slack.com/">Slack (#translation channel)</a>, <a href="https://github.com/mozilla-japan/translation">GitHub (mozilla-japan)</a>, <a href="https://groups.google.com/forum/#!forum/mozilla-translations-ja">Google Group (Mozilla.translations.ja)</a></li>
  <li>Current contributors: <a href="https://github.com/mfuji09">mfuji09</a>, <a href="https://github.com/hmatrjp">hmatrjp</a>, <a href="https://github.com/potappo">potappo</a>, <a href="https://github.com/dynamis">dynamis</a>, <a href="https://github.com/kenji-yamasaki">kenji-yamasaki</a></li>
 </ul>