Pro un explication, vide illac.

/* Le 3 de martio 2018. Converter un texto in elefen
   (lingua franca nova) del scriptura latin al cyrillic.

   Vide etiam https://rudhar.com/lingtics/intrlnga/scrptura/ia.htm

   Le 13 de martio 2018: secunde schema pro converter interlingua
   al scriptura grec.
  */

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

int conv_table[][3] =
{
   /* Cyrillico pro elefen primo, greco pro interlingua secundo.
          In le tertie columna un altere variante de greco.

      Vide https://rudhar.com/lingtics/uniclnkl.htm,
      https://unicode.org/charts/PDF/U0400.pdf e
      https://unicode.org/charts/PDF/U0370.pdf
    */
   /* Base value 1st column: 0410, 2nd & 3rd: 0390 */
   /* a */ { 0x00, 0x01, 0x01},
   /* b */ { 0x01, 0x02, 0x02},
   /* c */ { 0x0a, 0x0a, /* kappa, ma vide in basso pro theta */
                         0x69}, /* 3rd column: lunate sigma */
   /* d */ { 0x04, 0x04, 0x04},
   /* e */ { 0x05, 0x05, 0x05},
   /* f */ { 0x14, 0x16, 0x16},
   /* g */ { 0x03, 0x03, 0x03},
   /* h */ { 0x15, 0x07, 0x07},
   /* i */ { 0x08, 0x09, 0x09},
   /* j */ { 0x06, 0x17, 0x17},
   /* k */ {   -1, 0x0a, 0x0a},
   /* l */ { 0x0b, 0x0b, 0x0b},
   /* m */ { 0x0c, 0x0c, 0x0c},
   /* n */ { 0x0d, 0x0d, 0x0d},
   /* o */ { 0x0e, 0x0f, 0x0f},
   /* p */ { 0x0f, 0x10, 0x10},
   /* q */ {   -1, 0x18, 0x48}, /* 3rd column: archaic qoppa */
   /* r */ { 0x10, 0x11, 0x11},
   /* s */ { 0x11, 0x13, 0x13},
   /* t */ { 0x12, 0x14, 0x14},
   /* u */ { 0x13, 0x15, 0x15},
   /* v */ { 0x02, 0x15, 0x4c}, /* 3rd column: digamma */
   /* w */ {   -1, 0x19, 0x19},
   /* x */ { 0x18, 0x0e, 0x0e},
   /* y */ {   -1, 0x12, 0x18},
   /* z */ { 0x07, 0x06, 0x06},
   /* safety stop */ {-1, -1, -1},
};

static int convert (int c, FILE *fpi, FILE *fpo);
static enum {CYRILL = 0, GRECO = 1, GRECO2 = 2} conv_alphabet = CYRILL;
static int baseval = 0x0410;

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], "el") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-lel") == 0))
   {
      conv_alphabet = GRECO;
      baseval = 0x0390;
   }
   else if ((argc > 2 && argv[1][0] == '-' && strcmp(argv[2], "el2") == 0)
        ||
       (argc > 1 && strcmp(argv[1], "-lel2") == 0))
   {
      conv_alphabet = GRECO2;
      baseval = 0x0390;
   }

   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);
      else
         convert(c, fpi, fpo);
   }

   return 0;
}

static int convert (int c, FILE *fpi, FILE *fpo)
{
   if (!isascii(c) || !isalpha(c))
   {
      putc(c, fpo);
   }
   else
   {
      int tabval, index, extra_for_lowercase = 1;
      static int prev_was_c = 0;

      if (isupper(c))
      {
         index = c - 'A';
         extra_for_lowercase = 0;
      }
      else
      {
         index = c - 'a';
         extra_for_lowercase = 0x20;
         if (conv_alphabet == GRECO2)
         {
            switch (tolower(c))
            {
               case 'c':
                  extra_for_lowercase = -7;
                  break;
               case 'q':
               case 'v':
                  extra_for_lowercase = 1;
                  break;
            }
         }
      }

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

      tabval = conv_table[index][conv_alphabet];
      if (tabval < 0)
      {
         putc(c, fpo);
      }
      else
      {
         int n;

         tabval += baseval + extra_for_lowercase;

         if (conv_alphabet == GRECO)
         {
            if (isascii(c) && tolower(c) == 'c')
            {
               n = getc(fpi);

               if (strchr("eiyEIY", n) != NULL ||
                  (strchr("hH"    , n) != NULL && !prev_was_c))
               {
                  tabval = islower(c) ? 0x03b8 : 0x0398; /* theta */
               }
               ungetc(n, fpi);
            }
            else if (tabval == 0x03a2)
            /* For Interlingua:
               Final sigma (used for y) has no uppercase form. Use "upsilon with a hook"
               instead. See https://unicode.org/charts/PDF/U0370.pdf */
            {
               tabval = 0x03d2;
            }
         }
         else if (conv_alphabet == GRECO2)
         {
            if (c == 's')
            {
               n = getc(fpi);

               if (!isalpha(n))
               {
                  /* Final lowercase sigma as in real Greek */
                  tabval = 0x03c2;
               }
               ungetc(n, fpi);
            }
         }
         fprintf(fpo, "&#x%04x;", tabval);
      }
      prev_was_c = (isascii(c) && tolower(c) == 'c');
   }

   return 0;
}