Translations lost after upgrade

[C_T]

Hello, I upgraded to 2.9.0.beta14( fa8d6860b1 ) recently.
I saw an English message on the above of my page:

Let’s start the discussion! There are 20708 posts. Visitors need more to read and reply to – we recommend at least 1000000 posts. Only staff can see this message.

Previously, it was translated correctly. Now the translations are lost.
I found the issue might be related to this commit.

Update translations

It removed some translations from multiple languages.

Also, I found these strings displaying on my admin page are not translated:

Improved Notification Menu

We’ve updated our notification menu with improved filters to quickly see what notifications you have, and sort unread notifications at the top so you don’t miss anything.

and

Sidebar

We’ve overhauled Discourse’s navigation with a new sidebar. Check out the topic to learn more and how to enable it.

[gerhard (Gerhard Schlager)]

That happens whenever we make changes to English strings or add new strings. It takes some time until translators provide new translations. You can speed things up by heading over to https://translate.discourse.org and translating the untranslated strings yourself.

[C_T]

Thanks.
May I ask why it always happens? Why should translators provide existing translations again and again?

[Moin]

If a discourse feature changes the texts sometimes change. For example here was mentioned, that since you can now choose groups that are allowed to use whispers, the text telling this post is a private whisper only visible to moderators is not correct anymore. Then the new wording needs to be translated again. After the wording changed the translation must be changed too.

[C_T]

Thanks for the explanation.