fix: Prevent translation with identical source and target languages #1488

[mayank-sengar]

Description

Fixes issue #1488 where clicking "Translate" with identical source and target languages causes the spinner to spin endlessly without any user feedback.

Changes

This PR prevents translation requests when source and target languages are the same by:

1. Added validation in _toggleTranslation() - Before initiating translation, the method now checks if detectedFromLang equals detectedToLang. If they match, an alert is shown to the user with the message "You cannot translate to/from the same language" and the translation is not started.

2. Removed redundant UI warning - The plaintext warning "Translate To language is the same as the Source language" has been removed from the _statusWarning() method. This keeps the UI clean since the default language often matches the book's language anyway.

Problem Solved

• Before: Clicking "Translate" with same language selected → spinner spins indefinitely with no feedback
• After: Clicking "Translate" with same language selected → immediate alert prevents translation attempt

Files Changed

• src/plugins/translate/plugin.translate.js
  • Modified _toggleTranslation() method to add language validation
  • Modified _statusWarning() method to remove redundant message

Testing

To test this fix:

1. Open the BookReader and navigate to the Translate panel
2. Keep the "Translate To" and "Source" languages the same
3. Click the "Translate" button
4. Verify the alert appears with the message "You cannot translate to/from the same language"
5. Verify the spinner does not appear and translation does not start
6. Verify no plaintext warning appears at the bottom of the modal

Type of Change

• Bug fix (non-breaking change which fixes an issue)

[Copilot]

Pull request overview

Prevents users from initiating a translation when the selected source and target languages are the same, addressing the “endless spinner with no feedback” behavior reported in #1488.

Changes:

• Added a same-language guard in the Translate button handler to block starting translation and show an alert.
• Removed the previous plaintext same-language warning from the panel footer.

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[Copilot]

The new same-language guard runs unconditionally, which also blocks turning translation off if the user is currently translating and then changes the From/To dropdowns to the same language. In that state the button reads "Stop Translating", but clicking it will only alert and return, leaving translation stuck on. Consider only preventing the toggle when starting translation (e.g., when !this.userTranslationActive) or otherwise allow the stop action even if languages match.

Suggested change

	// Prevent translating when source and target are identical
	if (this.detectedFromLang === this.detectedToLang) {
	// Prevent starting translation when source and target are identical
	if (!this.userTranslationActive && this.detectedFromLang === this.detectedToLang) {