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No idea what Youtube does, but my first attempt would be:

• (Strip all illegal characters)
• (Censor profanity)
• If less than e.g. 600 characters, allow the whole comment.
• If any sentence ends between character 500 and 600, cut after the last sentence end.
• Otherwise, if any word ends between character 500 and 600, cut at the last one.
• Otherwise just cut at 600.

Then, as a final step:

• If the result of previous steps has more than e.g. 3 newlines, hide from the 3rd newline.

And a bonus feature: