I'd add a bit more detail.
There are indeed too few words in Hebrew for such things, so we have to overload words with several meanings. It might be a problem for other languages, too.
A good translation for "Post" is "רשומה". It definitely works well for something like a blog post, and it's also used in Flow. I'm not sure about which context you are talking here, however.
"Message" can be "מסר", and it works well in instant messages, for example in Twitter's private messages. But I'm not sure that it's very good for messages on talk pages, which are longer.
"Notice" is definitely "הודעה", not much to discuss here.
"Alert is definitely "התראה".
"Notification", in general, should be "הודעה", but when I was translating this a few months ago, I got some clarifications from Echo developers and liaisons and decided to give it the same name as "Alert" - "התראה". If it's changing now, I'd like to know how exactly.
It would super-ultra-extra-useful to have a glossary of all the Echo terms. Not just for Hebrew, but for all languages.
AFAIK, the Echo extension only uses three words: "Alerts", "Notices", and "Notifications".
IINM, "Posts" and "Messages" are not used in this extension.
"Notifications" is a general word that includes both "Alerts" and "Notices". This is actually the special page (Special:Notifications) that shows all kinds of notifications, i.e. both alerts and notices.
In Hebrew we have a problem, because "Alerts" means "התראות", "Notices" means "הודעות", and "Notifications" means either "הודעות" or "התראות". This results a confusing duplication, because this way the translation of "Notifications" must be similar to either "Alerts" or "Notices". Currently, it is similar to "Notices", as the special page in Hebrew is called "הודעות", while the blue badge is also called "הודעות".
Yeah, that's the issue more or less.
Months ago at the previous change I carefully went over all the messages and made it consistent. I had to be repetitive because we don't have enough varied words. I am fine with doing it one more time if needed, but I'd really appreciate a glossary that everybody can use. And of course it will be OK to update this glossary when needed.
If you translate "Notice" by "הודעה" and "Alert" by "התראה", that fine.
Translate "Notification" by "הודעה" is not that confusing, because Alerts are the most important Notifications.
Amir, speaking of glossary I think T129088 is for you. :)