Question: Is it possible to change strings (content and size) in Lua bytecode so that it will still be correct?
It's about translating strings in Lua bytecode. Of course, not every language has the same size for each word...

Answer: 
After some diving throught Lua source-code I found such a solution:

```
#include "lua.h"
#include "lauxlib.h"

#include "lopcodes.h"
#include "lobject.h"
#include "lundump.h"

/* Definition from luac.c: */
#define toproto(L,i) (clvalue(L->top+(i))->l.p)

writer_function(lua_State* L, const void* p, size_t size, void* u)
{
    UNUSED(L);
    return (fwrite(p,size,1,(FILE*)u)!=1) && (size!=0);
}

static void
lua_bytecode_change_const(lua_State *l, Proto *f_proto,
                   int const_index, const char *new_const)
{
    TValue *tmp_tv = NULL;
    const TString *tmp_ts = NULL;

    tmp_ts = luaS_newlstr(l, new_const, strlen(new_const));
    tmp_tv = &f_proto->k[INDEXK(const_index)];
    setsvalue(l, tmp_tv, tmp_ts);

    return;
}

int main(void)
{
    lua_State *l = NULL;
    Proto *lua_function_prototype = NULL;
    FILE *output_file_hnd = NULL;

    l = lua_open();
    luaL_loadfile(l, "some_input_file.lua");
    lua_proto = toproto(l, -1);
    output_file_hnd = fopen("some_output_file.luac", "w");

    lua_bytecode_change_const(l, lua_function_prototype, some_const_index, "some_new_const");
    lua_lock(l);
    luaU_dump(l, lua_function_prototype, writer_function, output_file_hnd, 0);
    lua_unlock(l);

    return 0;
}

```

Firstly, we have start Lua VM and load the script we want to modify. Compiled or not, doesn't matter. Then build a Lua function prototype, parse and change it's constant table. Dump Prototype to a file.

I hope You got that for the basic idea.
You can try using the decompiler [LuaDec](http://luadec51.luaforge.net/). The decompiler would allow the strings to be modified in generated Lua code similar to the original source.

[ChunkSpy](http://chunkspy.luaforge.net/) has [A No-Frills Introduction to Lua 5.1 VM Instructions](http://luaforge.net/docman/?group_id=83) that may help you understand the compiled chunk format and make the changes directly to bytecode if necessary.