I have implemented a maven plugin (which can be downloaded here  ) that helps you to find out what resource bundle keys are missing in different bundles. 

This can be useful when your web application suppose to support few languages. This plugin will ensure to find the missing keys in a maven compile time.

All you need to do is configure in your pom like below:

            <plugin>
                <groupId>com.maven.plugins</groupId>
                <artifactId>ResourceBundleValidator</artifactId>
                <version>1.0-SNAPSHOT</version>
                <configuration>
                    <resourceBundleDirectory>${basedir}/src/main/webapp/WEB-INF/resources</resourceBundleDirectory>
                    <resourceBundleGroups>
                        <property>
                            <name>labels</name>
                            <value>labels,labels_sv</value>
                        </property>
                        <property>
                            <name>messages</name>
                            <value>messages,messages_sv</value>
                        </property>
                    </resourceBundleGroups>
                </configuration>
                <executions>
                    <execution>
                        <phase>validate</phase>
                        <goals>
                            <goal>validateBundlesGoal</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

labels or labels_sv are the properties files which need to be in sync, as well as messages and messages_sv.

As soon as you run mvn clean package this plugin will validate the resource bundles and will ensure they are in sync if not the build will fail and the missing keys will be anounced in a the log.