It has nothing to do with your language or language files, your issue has to do with the fact that you have entered non-ASCII characters in your products (as opposed to using the HTML symbols, e.g. ® for ®) and since your v1.3.9d store is now using a version of PHP that interprets these characters using UTF-8 encoding unless otherwise specified.

That PHP change was one of the basic changes made to the core Zen Cart functionality starting in ZC1.5.1, to specify which character encoding to use.