fix(mkcontent): optimize paragraph text merging and language detection

[myhloli]

• Extract language detection to block level instead of line level
• Improve logic for handling Chinese, Japanese, and Korean languages
• Refactor code for better readability and performance
• Optimize handling of hyphenated words at line ends

Motivation

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Modification

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BC-breaking (Optional)

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Use cases (Optional)

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Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.