"mvn release:prepare" invokes maven-javadoc-plugin to generate javadoc. In my local environment, javadoc command uses HTML templates for Japanese unless -Duser.language=en option is given. So, I looked for the way to pass the option to javadoc and finally found it.
Additional JVM options can be passed to maven-javadoc-plugin by adding the block shown below to pom.xml.
<project>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<configuration>
<additionalJOption>-J-Duser.language=en</additionalJOption>
</configuration>
</plugin>
</plugins>
</build>
</project>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<configuration>
<additionalJOption>-J-Duser.language=en</additionalJOption>
</configuration>
</plugin>
</plugins>
</build>
</project>
However, note that, according to maven-javadoc-plugin FAQ, "Where in the pom.xml do I configure the Javadoc Plugin?", places to put configuration for maven-javadoc-plugin at vary depending on your purposes.