Captcha's aren't part of the Elgg core. There are however plugins which add this feature

• https://elgg.org/plugins/236859
• https://elgg.org/plugins/914006

These plugins are quit old and need to be updated for Elgg 6.x, you can ask the developers if they want to update their plugins or develop your own.

In PHP, text containing non-English characters may be perceived as having an increased number of characters due to how PHP processes string data. PHP utilizes a set of functions for string operations that are based on either single-byte or multi-byte encodings.

If PHP is configured to work with single-byte encodings (e.g., ISO-8859-1), each character is represented by one byte. However, many non-English characters (such as Cyrillic characters, ideograms, etc.) are encoded in UTF-8 using multiple bytes.

When handling strings that contain such characters without using multi-byte string functions (mbstring), PHP may incorrectly determine the length of the string and the number of characters it contains. For example, if you use the strlen() function to get the length of a string containing non-English characters in UTF-8, the result might show more characters than expected, as each multi-byte character will be considered as several single-byte characters.

We did use strlen instead of the mbstring version of that. I fixed it in https://github.com/Elgg/Elgg/pull/14715

So it should improve shortly.

Several of my users have reported this bug to me. They wanted to add more than one sub-comment to the same comment without leaving the page.

It will not work if a user add more than 1 sub comment on the same comment.

Apparently I already logged this issue https://github.com/Elgg/Elgg/issues/14690