I've tested some Apps in Marketplace. Some are good but others are not. For example, few of the portlets I tried would error out when I placed it in the home page and restarted liferay. Others has a logic to switch wording between different language in a jsp script even though there was also a Language.properties file. Others had liferay version (liferay-versions) set only to 6.1.1 without the "+" sign nor 6.2.0+ so I wasn't able to deploy it because of unsupported version.
It seems Liferay.com is checking if there are Language property file or not but just wondering how much testing is actually being done. For example, I saw several portlet that still had messages hardcoded.
BTW, the way to get message to jsp from Language.properties is as follows:
String tabNames = LanguageUtil.get(pageContext, "view.birthdays-events");
Also, it's good to localize date format in Language.properties file because different language usually have different date format display. Setting date format in system environmental is not enough because some portlets require different date format.
Example:
<%
String fmtString = LanguageUtil.get(pageContext, "birthday.date-format");
pageContext.setAttribute("dateFormat", fmtString);
%>
<td><fmt:formatDate value="${entry.birthDate}" pattern="${dateFormat}" /><br></td>
Signez svp dedans pour marquer ceci comme inadéquat.