TRANSLATE.BAT "This is my sentence."
Determine the length of %1
Then parse it and assign each character to a different variable.
Then do your conversion
Do the conversion one character at a time as you parse the input string. Then you only need just a few variables!
NOTE: It wouldn't take much more to read your sentence from a file to overcome the 500 character restriction and also convert <CR> and <LF> characters.
Please post your completed BAT file. I would very much like to have a copy. You should use the < pre - /pre > for all your code. It is very difficult to follow the way you have it.