How do you make Spring Boot default to a locale when a message key is missing?

How do I force Spring Boot to use a default locale when it can't find the message key in the user's given locale? I found this guide: https://www.baeldung.com/spring-boot-internationalization

I have the following:

@Configuration
public class WebConfig extends WebMvcConfigurerAdapter {
    @Bean
    public LocaleResolver localeResolver() {
        AcceptHeaderLocaleResolver lr = new AcceptHeaderLocaleResolver();
        lr.setDefaultLocale(Locale.ENGLISH);
        return lr;
    }

I have messages_es.properties, messages_en.properties (and many more).

I have the key oops-theme in messages_en.properties, but it is absent from all the other messages*.properties.

I set my browser to Accept-Language: es-ES,es;q=0.9,en-US;q=0.8,en;q=0.7.

Yet, when I access a JSP page with the oops-theme, it gives the exception

javax.servlet.jsp.JspTagException: No message found under code 'oops-theme' for locale 'es_ES'.

The JSP contains

<spring:message code="oops-theme"/>

How do I use a default locale for looking up messages when Spring Boot can't find them in the given locale?

Spring Boot 1.5.21.

Fill messages.properties with the messages for the fall-back text, e.g. in English.

Then don't write messages_en.properties, since the default messages are already in English.

Note that this is entirely independent of the setDefaultLocale(Locale.ENGLISH), as that simply states which Locale to use if the Accept-Language header is missing.

Example: You can write messages.properties in German, if you want, then have a messages_en.properties for English, and messages_es.properties for Spanish. You can then have setDefaultLocale(Locale.SPANISH), which means that if the browser doesn't specify a locale, you get Spanish, but for every message where a Spanish text hasn't been provided in messages_es.properties, you'd get the default German text from messages.properties.

You should read the javadoc of ResourceBundle and the PropertyResourceBundle subclass to learn how this all works.