You can hide the part in HTML comments:
BEFORE <!-- this is something I want to hide --> AFTER
Spoilers would be another option, but perhaps less clear in conveying the intention.
>! This is something I want to hide
I am not sure about people using mobile devices, but on desktop using comment replies doesn't seem that different for users with non-Latin usernames.
If I am in a comment where such user can be pinged, after I type @, I am offered the username(s) with non-standard characters.
For usernames which start with a Latin character, I am offered the username(s) ...
Since other answers mention HTML comments, let me clarify that using them is only a commonly employed custom on SE sites, which just abuses the fact that it works by accident.
In reality, the Stackexchange Markdown parser does not support HTML comments. What actually happens is that the initial < followed by unrecognized content is parsed as an invalid ...
Since Stack Exchange allows also some limited use of html, you can use <!---...---> to make a "comment" inside the post. Check the source code of this answer for an example.
I will add that I have seen warnings that mixing MarkDown and HTML too much might sometimes lead to problems, I do not know much about types of are known for html ...
% You can hide your drafts inside the math environment, using the percentage symbol. Click edit on my answer to see it in action.
% Assume the Riemann Hypothesis is false.
% Then clearly... hmm.
"posting something"... Do you mean a question or an answer? I occasionally to the following with my answers.
"Delete" it, until I am ready, then "undelete" it.
I have never tried this on my questions, but someone once told me that I cannot do that with questions.