Use ESPEAKNG_DEFAULT_VOICE instead of hard coded "en".

This will make it easier to set a default voice other than
English. This is important for cases when a language will fall back to
the default voice.

Some references to L('e', 'n') still need to be changed.

src/libespeak-ng/dictionary.c

 				}
 				*outptr = 0;
 				if (c == 0) {
-					if (strcmp(p_lang, "en") == 0) {
-						*p_lang = 0; // don't need "en", it's assumed by default
+					if (strcmp(p_lang, ESPEAKNG_DEFAULT_VOICE) == 0) {
+						*p_lang = 0; // don't need ESPEAKNG_DEFAULT_VOICE, it's assumed by default
 						return p;
 					}
 				} else

src/libespeak-ng/numbers.c

 		if (tr->translator_name == L('e', 'n'))
 			return; // we are already using English
 
-		SetTranslator2("en");
+		SetTranslator2(ESPEAKNG_DEFAULT_VOICE);
 		if (Lookup(translator2, &single_letter[2], ph_buf3) != 0) {
 			// yes, switch to English and re-translate the word
 			sprintf(ph_buf1, "%c", phonSWITCH);
 				// don't say "superscript" during normal text reading
 				Lookup(tr, modifier, capital);
 				if (capital[0] == 0) {
-					capital[2] = SetTranslator2("en"); // overwrites previous contents of translator2
+					capital[2] = SetTranslator2(ESPEAKNG_DEFAULT_VOICE); // overwrites previous contents of translator2
 					Lookup(translator2, modifier, &capital[3]);
 					if (capital[3] != 0) {
 						capital[0] = phonPAUSE;
 				ph_buf2[0] = 0;
 				if (Lookup(translator, alphabet->name, ph_alphabet) == 0) { // the original language for the current voice
 					// Can't find the local name for this alphabet, use the English name
-					ph_alphabet[2] = SetTranslator2("en"); // overwrites previous contents of translator2
+					ph_alphabet[2] = SetTranslator2(ESPEAKNG_DEFAULT_VOICE); // overwrites previous contents of translator2
 					Lookup(translator2, alphabet->name, ph_buf2);
 				} else if (translator != tr) {
 					phontab_1 = tr->phoneme_tab_ix;

src/libespeak-ng/readclause.c

 
 	if ((only == 0) && ((phonemes[0] == 0) || (phonemes[0] == phonSWITCH)) && (tr->translator_name != L('e', 'n'))) {
 		// not found, try English
-		SetTranslator2("en");
+		SetTranslator2(ESPEAKNG_DEFAULT_VOICE);
 		string = &single_letter[1];
 		single_letter[1] = '_';
 		if (LookupDictList(translator2, &string, phonemes, flags, 0, NULL) == 0) {
 			LookupDictList(translator2, &string, phonemes, flags, 0, NULL);
 		}
 		if (phonemes[0])
-			lang_name = "en";
+			lang_name = ESPEAKNG_DEFAULT_VOICE;
 		else
 			SelectPhonemeTable(voice->phoneme_tab_ix); // revert to original phoneme table
 	}
 		if (lang_name) {
 			SetWordStress(translator2, phonemes, flags, -1, 0);
 			DecodePhonemes(phonemes, phonemes2);
-			sprintf(buf, "[\002_^_%s %s _^_%s]]", "en", phonemes2, WordToString2(tr->translator_name));
+			sprintf(buf, "[\002_^_%s %s _^_%s]]", ESPEAKNG_DEFAULT_VOICE, phonemes2, WordToString2(tr->translator_name));
 			SelectPhonemeTable(voice->phoneme_tab_ix); // revert to original phoneme table
 		} else {
 			SetWordStress(tr, phonemes, flags, -1, 0);

src/libespeak-ng/speech.c

 
 	espeak_ng_STATUS status;
 	if (translator == NULL) {
-		status = espeak_ng_SetVoiceByName("en");
+		status = espeak_ng_SetVoiceByName(ESPEAKNG_DEFAULT_VOICE);
 		if (status != ENS_OK)
 			return status;
 	}

src/libespeak-ng/translate.c

 			if (unpron_phonemes[0] == phonSWITCH) {
 				// change to another language in order to translate this word
 				strcpy(word_phonemes, unpron_phonemes);
-				if (strcmp(&unpron_phonemes[1], "en") == 0)
+				if (strcmp(&unpron_phonemes[1], ESPEAKNG_DEFAULT_VOICE) == 0)
 					return FLAG_SPELLWORD; // _^_en must have been set in TranslateLetter(), not *_rules which uses only _^_
 				return 0;
 			}
 
 				new_language = (char *)(&p[1]);
 				if (new_language[0] == 0)
-					new_language = "en";
+					new_language = ESPEAKNG_DEFAULT_VOICE;
 
 				switch_phonemes = SetTranslator2(new_language);
 

src/libespeak-ng/voices.c

 	f_voice = fopen(buf, "r");
 
         if (!(control & 8)/*compiling phonemes*/)
-            language_type = "en"; // default
+            language_type = ESPEAKNG_DEFAULT_VOICE; // default
         else
             language_type = "";