For localization, you can change any text fields to i18n labels by using the small gear icon:

For example, let's look at the following two i18n files:

#demo_en.properties
label.text=LABEL
tf1.text=HELLO

#demo_fr.properties
label.text=étiqueter
tf1.text=bonjour

Let's apply them to our small UI. First, we change all text values to i18n variables:

And now, we can select English and French versions and preview the application without even compiling any code!

Here is the English version:

Here is the French version:

This way, you can be sure that your UI looks good in all supported localizations.

In Java code, you can set the resource bundle used for a specific FXML localization in the Loader API:

// for current system locale
loader.setResources(ResourceBundle.getBundle("chapter4.demo")); 
// for specific locale
loader.setResources(ResourceBundle.getBundle("chapter4.demo", new Locale("fr", "FR")); 

Here, chapter4 is a package name and demo is the first part of the filename (demo_fr.properties).