Improve language detection for forwarded emails where the forwarder adds no content

Summary: When an email is forwarded to our support address with no text from the forwarder, Intercom detects the conversation language as English, based on the forwarder's signature and the mail client's forwarding boilerplate. The forwarded content and the subject line are in German. Language detection should ignore boilerplate and signatures and weigh the forwarded message's actual content. 

Current behavior:

1. A teammate forwards a German email to our support inbox, adding nothing above the forwarded block.
2. The conversation is created with a German subject and a German forwarded body.
3. Intercom sets the detected language to English. The likely trigger is the English signature and the English wrapper ("Forwarded message", "From / Date / Subject / To").

Expected behavior: The conversation is detected as German, since the subject and the substantive body text are German.

Why it matters:

• Wrong language detection breaks everything downstream that depends on it: routing and assignment rules, language-based workflows, Fin's response language, and macro/snippet selection.
• Forwarding is a standard internal handoff pattern, so this is systematic, not an edge case.
• Non-English customers get routed to English queues or answered in the wrong language.