As visible on these snippets from the day of the original ABC broadcast : 1 and 2, the title used was only "It" and not "Stephen King's It". Consequently, the original title and the English language title should be "It".

I have left a message to a German moderator for the German title.

The title from the day of the original ABC broadcast is irrelevant if it is absolutely not common. The main task of a movie database is to find films with the common film title. Google Search: Stephen Kings Es Internet movie database (IMDb): Stephen Kings Es (tt0099864) Amazon (de): Stephen Kings Es Amazon (com): Stephen King, IT! Filmstarts.de: Stephen Kings Es Kino.de: Stephen King's It Moviepilot.de: Stephen Kings Es wikipedia.org: Titel: Stephen Kings Es; Originaltitel: Stephen King’s It etc. In addition, IMDb is still the largest movie database, it should generally serve as a reference.

As per our rules, the original title should always be the title used for the first release, and a language title should always be the title used for the first release in a country speaking that language.

By the way, IMDB also list it under the title "It" : https://i.imgur.com/YuOGjpP.png

I am familiar with the TMDB naming rules. The title “Stephen King’s It” is common on German movie portals and on the Internet. IMDb seems to take this into account: https://imgur.com/a/2X2XST1 Maybe TMDB should take an example ;)

As indicated above, my answer concern the original title.

For the German title, you will have to wait for the answer of a German moderator.

Okay... thanks.