YATCP allows you to have threaded comments (i.e. reply to existing comments) in your wordpress.
Mail me at firstname.lastname@example.org. Include at least a Bug report (Patches are highly welcome), and we see what we can do.
YATCP is made available under GPL v2.
Since Version 0.6 YATCP uses the Standard WordPress i18n mechanism to provide translated texts. If you have configured your WordPress correctly and there already is a translation for your language YATCP should pick that up automatically. If your're getting english texts, most probably no translation exists for your language. Have a look at the Translation Page for further information.
YATCP was made to be adaptible to the template is has to live in, so you can change it nearly in any way. Please refer to the Customization Description for more detail on that Issue.
I have tested YATCP on WordPress 2.0.7, 2.1, 2.2.1, 2.3.x and 2.5 successfully. I'm not quite aware since when the comment_parent column is present in WordPress, but your Version has at least to have that Database Column in order for YATCP to work correctly.