1. Create files for your messages

 messages.properties   
 messages_en.properties
 messages_fr.properties   
 ...

2. Write messages in this files like this

header.label.title=Title

3. Configure path to this files (in this case in folder D:/project/messages) in application properties like:

messages.basename.path=D:/project/messages/messages

4. Configure MessageSource

@Value("${messages.basename.path}")
private String messagesBasename;

@Bean
public MessageSource messageSource() {
       ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
       messageSource.setFallbackToSystemLocale(false);
       messageSource.setBasenames("file:" + messagesBasename);
       return messageSource;
   }

5. Use messages on pages

<p th:text="#{header.label.title}">Title</p>

Write message in messages.properties

welcome.message=Hello, {0}!

Replace {0} with the user name inside thymeleaf tag

<h3 th:text="#{welcome.message(${some.variable})}">Hello, Placeholder</h3>