Pro un explication, vide illac.

/* Le 4 de julio 2018. Converter un texto in elefen
   (lingua franca nova), interlingua etc. del scriptura
   latin al devanagari, e altere scripturas de India.
  */

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int conv_table[][2] =
{
   /* Vide https://rudhar.com/lingtics/uniclnkl.htm,
      https://unicode.org/charts/PDF/U0900.pdf Devanagari
      https://unicode.org/charts/PDF/U0980.pdf Bengali
      https://unicode.org/charts/PDF/U0A00.pdf Gurmukhi
      https://unicode.org/charts/PDF/U0A80.pdf Gujarati
    */
   /* For vowels: dependent first, independent next */
   /* a */ { 0x3e, 0x06}, /* Long aa = vowel a, no vowel = absence (not short a),
                             to avoid excessive use of virama *.
   /* b */ { 0x2c, -1  },
   /* c */ { 0x1a, -1  }, /* Misused palatal consonant for this */
   /* d */ { 0x26, -1  },
   /* e */ { 0x47, 0x0f},
   /* f */ { 0x2b, -1  }, /* Misused ph for this */
   /* g */ { 0x17, -1  },
   /* h */ { 0x39, -1  },
   /* i */ { 0x3f, 0x07},
   /* j */ { 0x1c, -1  }, /* Misused palatal consonant for this */
   /* k */ { 0x15, -1  },
   /* l */ { 0x32, -1  },
   /* m */ { 0x2e, -1  },
   /* n */ { 0x28, -1  },
   /* o */ { 0x4b, 0x13},
   /* p */ { 0x2a, -1  },
   /* q */ { 0x18, -1  }, /* Misused gha for this */
   /* r */ { 0x30, -1  },
   /* s */ { 0x38, -1  },
   /* t */ { 0x24, -1  },
   /* u */ { 0x41, 0x09},
   /* v */ { 0x2d, -1  }, /* Misused bh for this */
   /* w */ { 0x35, -1  },
   /* x */ { 0x16, -1  }, /* Misused kh for this */
   /* y */ { 0x40, 0x08}, /* Misused long ii for this */
   /* z */ { 0x5b, -1  },
   /* safety stop */ {-1, -1},
};

static int convert (int c, FILE *fpi, FILE *fpo);
static int baseval = 0x0900;
static int prev_was_cons = 0;

int  main (int argc, char **argv)
{
   FILE *fpi = stdin, *fpo = stdout;
   int c;
   int intag = 0, inentity = 0;

   if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "Deva") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-sDeva") == 0))
   {
      baseval = 0x0900;
   }
   else if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "Beng") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-sBeng") == 0))
   {
      baseval = 0x0980;
   }
   else if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "Guru") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-sGuru") == 0))
   {
      baseval = 0x0a00;
   }
   else if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "Gujr") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-sGujr") == 0))
   {
      baseval = 0x0a80;
   }
   else if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "Tibt") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-sTibt") == 0))
   {
      baseval = 0x0a80;
   }

   while ((c = getc(fpi)) != EOF)
   {
      if (!intag && c == '<')
         intag = 1;
      else if (intag && c == '>')
         intag = 0;
      else if (!inentity && c == '&')
         inentity = 1;
      else if (inentity && c == ';')
         inentity = 0;

      if (intag || inentity)
      {
         putc(c, fpo);
         prev_was_cons = 0;
      }
      else
      {
         convert(c, fpi, fpo);
      }
   }

   return 0;
}

static int convert (int c, FILE *fpi, FILE *fpo)
{
   if (!isascii(c) || !isalpha(c))
   {
      putc(c, fpo);
      prev_was_cons = 0;
   }
   else
   {
      int tabval, index;

      index = (c = tolower(c)) - 'a';

      /* Safety first */
      if (index >= 26)
          index = 26;

      tabval = conv_table[index][0];
      if (!prev_was_cons && strchr("aeiouy", c) != NULL)
         tabval = conv_table[index][1];

      tabval += baseval;
      fprintf(fpo, "&#x%03x;", tabval);
      prev_was_cons = (strchr("aeiouy", c) == NULL);
   }

   return 0;
}