If you look at the subforum for RichMemo there is a decent list of bugs, the largest of which include composition and entry issues with left-to-right scripts. Unfortunately if the OP needs a component which works with every language on Earth they will either need to use Window's native RichEdit or GTK's equivalent.
The biggest issue here, is that RichMemo is using Window's native RichEdit...
However, the problems with left-to-right scripts you're referring to are caused by the fact the entries of left-to-right characters are done, via non-standard Windows means.
(Typically Windows requires a user to install a keyboard layout for an appropriate language and use it for entry).
imho, there's certainty an issue when people are trying to compare lzRichEdit vs RichMemo.
BOTH are based on Windows native RICHEDIT component
Thus, in the end would have exactly the same set of issues anyway. (imposed by Windows native RICHEDIT component).
Another problem, is that somehow the fact of "rich-text" being a complex task fades.
If, K-Memo is not enough (and so far, it's the most advanced open-source and free component to use for rich-text editing), I'd certainly recommend to turn to a commercial options... such as
Rich Text Component.