Fix/ck5 mt incompatibility textpartlanguage

[carla-at-wiris]

Description

When MathType and TextPartLanguage plugins are included in the same build, the use of the MathType plugin will result in an error. There's a conflict on the Wiris end and regarding the getLanguage() method.

This PR fixes the conflict by returning the editor's UI language when faced with a language object without the ui property.

• Related Kanbanize Card: Link to KB-45315
• Related GitHub Issue: Closes [CKEditor 5] TypeError thrown when using MathType in combination with TextPartLanguage #910

Type of Change

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

Checklist

• My code follows the style guidelines of this project ( Run yarn lint to check)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• New and existing unit tests pass locally with my changes

How should be tested?

1. Add the TextPartLanguage plugin to the CKEditor5 HTML demo. Check the documentation for guidance.
2. Insert or modify a formula via MathType

[xjiang-at-wiris]

Works perfectly!

[Copilot]

Pull Request Overview

This PR fixes a compatibility issue between MathType and TextPartLanguage plugins in CKEditor5 by resolving a conflict in the getLanguage() method. When these plugins are used together, the MathType plugin encounters an error due to how language objects are handled.

Key changes:

• Replaces direct property access with safer Object.prototype.hasOwnProperty.call() method
• Returns editor's UI language when language object lacks the ui property instead of returning the entire object

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