edit sis files

You can't edit a .sis file directly. Search for "unmakesis": http://www.allaboutsymbian.com/forum/search.php?

Note, however, that it is only useful to unpackage some .sis files (not all), and after that you'd have to patch the binaries (with limited chances of success for changing the language of an application), or have the language resource file sources and tools (from the SDK) to recompile them, then recreate the .sis file, etc.

In other words, if you need an application in a different language, your best bet is to contact the developer of the application.