maintainability: pass current_alphabet as argument to TranslateLetter() instead of using global

src/libespeak-ng/numbers.c

 	return 0;
 }
 
-int TranslateLetter(Translator *tr, char *word, char *phonemes, int control)
+int TranslateLetter(Translator *tr, char *word, char *phonemes, int control, ALPHABET *current_alphabet)
 {
 	// get pronunciation for an isolated letter
 	// return number of bytes used by the letter

src/libespeak-ng/numbers.h

 #ifndef ESPEAK_NG_NUMBERS_H
 #define ESPEAK_NG_NUMBERS_H
 
+#include "translate.h"
+
 #ifdef __cplusplus
 extern "C"
 {
 void SetSpellingStress(Translator *tr, char *phonemes, int control, int n_chars);
 int TranslateRoman(Translator *tr, char *word, char *ph_out, WORD_TAB *wtab);
 int TranslateNumber(Translator *tr, char *word1, char *ph_out, unsigned int *flags, WORD_TAB *wtab, int control);
-int TranslateLetter(Translator *tr, char *word, char *phonemes, int control);
+int TranslateLetter(Translator *tr, char *word, char *phonemes, int control, ALPHABET *current_alphabet);
 
 
 #ifdef __cplusplus

src/libespeak-ng/translate.c

 		capitals |= 4; // speak charater code for unknown letters
 
 	while ((*word != ' ') && (*word != 0)) {
-		word += TranslateLetter(tr, word, phonemes, capitals | non_initial);
+		word += TranslateLetter(tr, word, phonemes, capitals | non_initial, current_alphabet);
 		posn++;
 		non_initial = true;
 		if (phonemes[0] == phonSWITCH) {
 			if (posn > 0)
 				non_initial = true;
 
-			wordx += TranslateLetter(tr, wordx, unpron_phonemes, non_initial);
+			wordx += TranslateLetter(tr, wordx, unpron_phonemes, non_initial, current_alphabet);
 			posn++;
 			if (unpron_phonemes[0] == phonSWITCH) {
 				// change to another language in order to translate this word

src/libespeak-ng/translate.h

 	int flags;
 } ALPHABET;
 
-extern ALPHABET *current_alphabet;
 // alphabet flags
 #define AL_DONT_NAME    0x01 // don't speak the alphabet name
 #define AL_NOT_LETTERS  0x02 // don't use the language for speaking letters