You should not use I18N.tr in a static variable. This method must be called when the value is requested. However in order to register this string in translation dictionary (.po files) you should use in this case I18n.marktr() http://xnap-commons.sourceforge.net/gettext-commons/apidocs/org/xnap/commons/i18n/I18n.html#marktr%28java.lang.String%29
```java
protected static final String UNIT_OF_MEASURE = I18n.marktr("Unit of measure"); 
```