core/play/src/main/java/play/i18n/MessagesApi.java
public Boolean isDefinedAt(play.api.i18n.Lang lang, String key) {
    return messages.isDefinedAt(key, lang);
  }
public Boolean isDefinedAt(play.api.i18n.Lang lang, String key) {
    return messages.isDefinedAt(key, lang);
  }
Check if a message key is defined.

@param lang the message lang
@param key the message key
@return a Boolean