I am using spring boot with thymeleaf. This is my project structure:

And this
Refer the official documentation for spring boot
http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#howto-convert-an-existing-application-to-spring-boot
It says
Static resources can be moved to
/public(or/staticor/resourcesor/META-INF/resources) in the classpath root. Same formessages.properties(Spring Boot detects this automatically in the root of the classpath).
So you should create ur internationalization file as messages.properties and place in the root classpath.
Or you can also edit the default location to a more proper location by adding this entry in the application.properties file
#messages
spring.messages.basename=locale/messages
so you can store your files in the locale folder inside resources folder, with the name messages.properties or in any specific language.