Editing a language file is actually pretty easy ...

All text is defined in the language files, and you can use your templates and overrides to make your custom changes so that they do not get overwritten on an upgrade ...

If you have trouble figuring out where the text is located, you can use the Tools ... Developer's Tool Kit ...

In the bottom input box enter a part of the text you are looking for, and select the Catalog from the dropdown ... Click Search ...

This will locate for you the language file(s) where the define statement(s) are setup ...

Other text, is located in the Define Pages ... this is usually a block of text that also can be copied to your templates and overrides directory and customized to meet your needs ...