Mail.app Conversation/Threading issues like RES: RES: RES:
The threading does happen based on subject.  Re: is ignored; so Re: Re: Re: foo is the same as foo, and the same as Re: Re: foo.  That's why it works.

What is RES:?  Is it Re: in a different language?  That's why it's breaking - because RES is not being ignored and therefore looks like a different subject.

The person you're conversing with needs to use Re:, not RES:.  RES: may be inserted automatically by their email client; in which case you may need to find a different way to group the emails, perhaps by sender.

There are tools that can help, though.  With MailTags (http://www.indev.ca/MailTags.html) you can group/organize your email much differently or simply use the subject renaming feature.  With MailActOn (http://www.indev.ca/MailActOn.html) you can match a rule that moves the email to a specific folder ignoring the "RES" part, etc.  There are many options.

No, I am not affiliated with Indev, I just use and love their products.  :-)
Best of luck.