Closes #87 TTS voice language

Source/EPUBCore/FREpubParser.swift

@@ -253,7 +253,8 @@ class FREpubParser: NSObject, SSZipArchiveDelegate {
             }
 
             if tag.name == "dc:language" {
-                metadata.language = tag.value ?? ""
+                let language = tag.value ?? metadata.language
+                metadata.language = language != "en" ? language : metadata.language
             }
 
             if tag.name == "dc:creator" {

Source/EPUBCore/FRMetadata.swift

@@ -85,7 +85,7 @@ class FRMetadata: NSObject {
     var creators = [Author]()
     var contributors = [Author]()
     var dates = [Date]()
-    var language = "en"
+    var language = "en-US"
     var titles = [String]()
     var identifiers = [Identifier]()
     var subjects = [String]()

Source/FolioReaderAudioPlayer.swift

@@ -348,6 +348,8 @@ class FolioReaderAudioPlayer: NSObject, AVAudioPlayerDelegate, AVSpeechSynthesiz
 
         let utterance = AVSpeechUtterance(string: text)
         utterance.rate = utteranceRate
+        utterance.voice = AVSpeechSynthesisVoice(language: book.metadata.language)
+
         if(synthesizer.speaking){
             synthesizer.stopSpeakingAtBoundary(AVSpeechBoundary.Word)
         }