How to provide your own custom MessagesApi in PlayFramework when using a "base" subproject for branded projects.

ApplicationController.scala

package com.wieck.base
package controllers

import play.api.{Application, Play, mvc, i18n}, mvc._, i18n._

abstract class ApplicationController extends Controller {

  private val messagesApiCache = Application.instanceCache[BaseMessagesApi]
  implicit def messages(implicit lang: Lang): Messages = new Messages(lang, messagesApiCache(Play.current))
  
}

BaseMessagesApi.scala

package com.wieck.base

import javax.inject.Inject
import play.api.{Configuration, Environment, i18n}, i18n.{Langs, DefaultMessagesApi}

class BaseMessagesApi @Inject() (environment: Environment, configuration: Configuration, langs: Langs) extends DefaultMessagesApi(environment, configuration, langs) {
  override protected def loadAllMessages: Map[String, Map[String, String]] = {
    langs.availables.map(_.code).map { lang =>
      (lang, loadMessages("messages." + lang))
    }.toMap
      // The following line is the only deviation from the DefaultMessagesApi. It loads your base messages,
      // then merges your customized messages over them.
      .+("default" -> (loadMessages("messages.base") ++ loadMessages("messages")))
      .+("default.play" -> loadMessages("messages.default"))
  }
}