If answering the question does not require code then it is fine to close it. But in any other way no.
C++ is not Delphi and translating even simple API calls from C++ to Delphi side may be hard job for some people.
I would not close it as duplicate, even though Delphi answer provided does not have any code. Someone might come by and provide some relevant Delphi code. If questions is closed he cannot and posting Delphi code under C++ duplicate would not go well.
Or in another words. If the only thing wrong with the question is that it is API question that already has answer in another language tag then it should not be closed as duplicate.
If it is bad question, and dupehammer is just a way of closing bad question quickly even if dupe target is not "perfect" then I would say go with it.
Why closing good API/frameworks as dupes in another languages is bad.
While API is the same languages are different. That should be sufficent reason enough. People asking question and tagging it with specific language tag don't have to be familiar with other language.
Understanding API and explanation about API is one thing, understanding code written in different language is another.
SO allows beginner questions if they are good enough. Learning coding is hard enough when you are fresh. Should we force people learning their first language to learn another too?
Closing such questions as dupes, we are not only doing that to OP, but to other developers that might find anser in particular language useful. We are also forcing them to use language they might not know.
In this case it is about closing Delphi question as C++ dupe. What if first question asked would be Delphi one. How would you feel that C++ question about some API gets closed as Delphi dupe. Or Fortran. Or any other language capable of interacting with said API.
What if non C++ dev can provide better answer than the ones posted in C++ question. If Delphi question gets closed he cannot do that. Would it be acceptable to post Delphi code with API explanation under C++ tag. Would it be acceptable to post Fortran code there.
Would it be acceptable for Delphi question about FireMonkey API to be answered with C++ Builder code or closed as C++ Builder dupe. I don't think so.
Android SDK API uses Java. There are tools offering Pascal, Swift and C# variants. Would it be acceptable to use any of those for answering any Android API question?
Providing code opens up doors to copy-paste programming. Providing any code does. How do you know C++ beginner would not take C++ code and use it without understanding. It is not our job to tell people how should they use materials they find here. It is their responsibility.
We ask people to understand code they will use instead of just copy pasting it. How can they understand code in language they don't understand. Even if you do transtale code by yourself you can make subtle errors with translation. Translating does not imply understanding.
With this kind of practice we are discriminating developers from languages that are not primary for any given API.
Commenting or adding link to the other question in your own elaborate answer, pointing to additional read in another question would be fine. Closing question would not.
Making this kind of dupe closing proper practice we open up doors to total mess.