Repaired the polish language file

Many characters in the polish language file were invalid, so they are fixed now.
enhance escapeshellarg for unicode characters

By default escapeshellarg will strip any unicode characters.
Thus, replacing escapeshellarg() with mb_escapeshellarg().

Hard-coded accented characters should be avoided in php files
due to various text editors which might overwrite them.
The safer way to represent accented characters is to use chr() function.
Thus, added the remove_accents() dfunction.