SpringBoot: accessing custom i18n validation message programmatically

Simple problem here: I'm working on a Spring Boot website with 3 language versions, I have internationalized the messages using messages_xx.properties where xx is the language prefix, and the following code:

    @Bean
    public LocaleResolver localeResolver() {
        SessionLocaleResolver slr = new SessionLocaleResolver();
        slr.setDefaultLocale(Locale.US);
        return slr;
    }

    @Bean
    public LocaleChangeInterceptor localeChangeInterceptor() {
        LocaleChangeInterceptor lci = new LocaleChangeInterceptor();
        lci.setParamName("lang");
        return lci;
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(localeChangeInterceptor());
    }

This works fine. Then, to keep things simple, I have also created ValidationMessages_xx.properties for validation and it works fine, too. The problem arises when I need to access some message from ValidationMessages programmatically, for instance:

        if (!userDto.getPassword().equals(userDto.getRepeatPassword())) {
            bindingResult.rejectValue("repeatPassword", "error.repeatPassword", "custom.validation.passwordsDontMatch.message");
        }

The probliem lies in third parameter for rejectValue - I need to somehow substitute a message from the ValidationMessages here. But the code shown above only returns the literal string "custom.validation.passwordsDontMatch.message", even though the Idea editor shows it "sees" the correct message from ValidationMessages here when I put the cursor over the string. This doesn't work either:

bindingResult.rejectValue("username", "error.username", "${custom.validation.passwordsDontMatch.message}");

How can I access that custom.validation.passwordsDontMatch.message string from a Java class?

ResourceBundle bundle = ResourceBundle.getBundle("ValidationMessages", LocaleContextHolder.getLocale());
String value = bundle.getString("custom.validation.passwordsDontMatch.message");