AppConfiguration.kt

@Configuration
open class AppConfiguration {

    // ... Abridged for readability's sake

    @Bean
    open fun vaadinServlet() = CustomVaadinServlet{ event: SessionInitEvent ->
        event.session.addBootstrapListener(object : BootstrapListener {
            override fun modifyBootstrapFragment(response: BootstrapFragmentResponse) {
                // NOP, this is for portlets etc
            }
            override fun modifyBootstrapPage(response: BootstrapPageResponse) {
                response.document.child(0).attr("lang", "fr")
            }
        })
    }
}

class CustomVaadinServlet(private val listener: (SessionInitEvent) -> Unit) : SpringVaadinServlet() {
    override fun servletInitialized() {
        super.servletInitialized();
        service.addSessionInitListener(listener)
    }
}