We can also override a Language.properties files from a hook.The process is almost similar to the ones described in this post.
All you need to do is create a Language file for the language whose messages you want to customize and then refer to it from your liferay-hook.xml.
Here are the steps :
1) Specify below property in liferay-portlet.xml.
2) Create Lanuguage.properties and Language_en.properties under WEB-INF/src/content.
Provide labels you want to override .