I created a Spring Boot web application that uses Thymeleaf as the template engine. I configured the MessageSource to look for messages in a subfolder:
MessageSource
The message source be relative to classpath:
messageSource.setBasename(" classpath:i18n/messages");
Here is a tutorial I referenced for thymeleaf and spring = http://www.thymeleaf.org/doc/tutorials/2.1/thymeleafspring.html