#C, 120 Bytes
This needs to be golfed
j;f(char*m){for(j=0;*m;m++)putchar(!isalpha(*m)?*m:*m==32?*m:"Blet"[j]),j=!isalpha(*(m+2))?j==0?1:3:*m==32?0:j^2?j+1:j;}
main(c,v)char**v;{
f("The End is near Fellows!");puts("");
f("We praise the Goat God!");puts("");
f("Jimmy's test");puts("");
f("0te_st1");puts("");
f("_012345678");
}
Blt Blt Bt Blet Bleeeet!
Bl Bleeet Blt Blet Blt!
Bleet't Blet <------ Not so sure about this
0lt_Tt1 <------ This is hard :-/
_012345678