Pre-flight Checklist

• I have searched existing issues, and this hasn't been requested yet.

Problem or Motivation

The labels config translates most of the prebuilt chat, but some strings bypass it and stay in English whatever the app passes. In @copilotkit/react-core@1.79.1 (unchanged on main), with every label set to German:

• The reasoning header shows "Thinking…" while the model reasons and "Thought for a few seconds" / "Thought for 12 seconds" / "Thought for 2m 5s" afterwards (CopilotChatReasoningMessage.tsx). The duration formatter is English-only.
• Screen readers announce English names: "Remove attachment" and "Play video" (CopilotChatAttachmentQueue.tsx), "Close preview" (Lightbox.tsx), "Slash commands" plus the visible "No commands found" (CopilotChatInput.tsx), "Close" and "Open threads" (CopilotModalHeader.tsx), and "Copilot chat sidebar" (CopilotSidebarView.tsx).

Vue has the same strings. Angular hard-codes "Thinking…" / "Thought for …", "Remove attachment", "Close Copilot chat" and "Close Copilot sidebar".

The only way around it today is to replace slots. For the reasoning header even that falls short: the header slot gets label and isStreaming, but not the elapsed time, so a custom header can't say "Thought for 12 seconds" in another language without re-implementing the timer.

With German labels on 1.79.1 (repro: https://github.com/Mordris/copilotkit-ui-repro), the reasoning headers on the left and right stay English:

🎤 Tell us your idea

Route these strings through labels, the way #6771 did for the add-menu tooltip:

"Thought for N seconds" carries a number and plural rules, which a plain string label can't express. I'd keep its current English output as the default and pass elapsedSeconds to the reasoning header slot, so an app can render its own text with correct plurals:

<CopilotChat
  messageView={{
    reasoningMessage: {
      header: ({ isStreaming, elapsedSeconds, ...props }) => (
        <CopilotChatReasoningMessage.Header
          {...props}
          isStreaming={isStreaming}
          label={isStreaming ? "Denkt nach…" : `${Math.round(elapsedSeconds)} Sekunden nachgedacht`}
        />
      ),
    },
  }}
/>

Everything is additive: same English defaults, no visual change. Each framework keeps its own default text where it differs today, as it already does for chatInputToolbarAddButtonLabel. The A2UI renderer's own strings ("Building interface", "Retrying generation…", "Filter options...") are left out, since the A2UI catalog is being reworked in #7819 / #7829.

Icon-only buttons with no accessible name at all (send/stop, add, voice, scroll to bottom, branch navigation) are a separate bug: #7862.

Would you like to work on this?

• Yes, I’d love to work on it!
• I’m open to collaborating but need guidance.
• No, I’m just sharing the idea.