Any clues where these texts are coming from would be appreciated. I might want to dive into it in order to get this fixed, but I got no clue where to start looking.

I tested it and it seems to be working fine for me. Can you double-check if it's still as you describe?

Hi @Lydia_Pintscher , thanks for testing. I retested it. The problem remains the same, when my language preferences are set to Dutch (nl). [https://www.wikidata.org/wiki/Special:SetEntitySchemaLabelDescriptionAliases/E105/de] then gives me 'You are setting label, description and aliases in Duits for Schema E105.' on top of the page, but 'Het label van het schema in het Engels' in grey in the box for the label.
The first sentence should be in Dutch, the second one should mention 'Duits' (instead of 'Engels').
When I switch to English or German as language, it seems to be working fine.

I got no clue where these texts come from, so I have no idea where to find a place to correct this.

Thank you!
Campers: Maybe you can investigate/provide additional info?

The translations are coming from translatewiki. The offending keys seem to be entityschema-label-edit-placeholder, entityschema-description-edit-placeholder, entityschema-aliases-edit-placeholder.

Thank you!

Thank you both. Where can I find this translatewiki so I can put in a language reference, instead of the (hardcoded?) 'Engels'?
And which placeholder is for the introducing text ('You are setting label, description and aliases in <Language> for Schema <Entitynumber>')? That one is in English, should be translated (made available in Dutch too).