This tips is easy to find nevermind I post it to have a note about it.
For example in a JDialog or JFrame constructor :

this.setLocationRelativeTo(null);

Easy isn’it ?

Here a small code example to manage messages into java application. This mono instance class allow you to load messages according language. You could also replace token into messages with dynamical argument.
You can find java class at the end of this post.
This class does not contain a lot of code. The next instruction allow to [...]