That's the placeholder icon for when the site icon is not configured. It's meant to use the logo of the wiki. It can be configured by setting $wgEchoNotificationIcons['site']['url']. See here and here for how it's set in WMF production.

If you want TranslateWiki to have a nice welcome icon, you should tag this with translatewiki.net and ask them to set this config setting. If you want the fallback for the welcome icon to be nicer, I'd be happy to incorporate a nicer icon if you design one (or someone else does).

Just quickly created an icon using the TWN logo since that seems to be the format.
Created in 20x20 and 30x30px in color (their logo colors), grayscale (WMF colors) and mono (single WMF color) to select from depending on what the convention is for showing site logo icons on Echo:

@Nikerabbit Is this important for you or the TWN community? If so, could you make the config change described above, or let me know where I can do it?

There is a TranslatewikiSettings.php in the translatewiki repository.

I guess I should do the change, as I need to upload the files to suitable location first.

Change 754458 had a related patch set uploaded (by Nikerabbit; author: Nikerabbit):

[translatewiki@master] Set custom site icon for echo notifications

https://gerrit.wikimedia.org/r/754458

Change 754458 merged by jenkins-bot:

[translatewiki@master] Set custom site icon for echo notifications

https://gerrit.wikimedia.org/r/754458