API would certainly not be an appropriate tag. That's about as vague as you can possibly get.
I would leave it tagged as is. The code sample in the question is C#, he will be looking for answers using C#. Using the criteria that it's "just coding to an API, the language is irrelevant" would mean that most questions would have their tags removed unless it was specific about language constructs or syntax, and that is not what we're after, either for the user asking about C# or the person searching in google and including C#.
We don't want to add tags where they are not appropriate (ie., spamming a question with tags for languages that are not actually involved), but removing tags for involved languages should also be discouraged.
From your comment in the question
This is language agnostic. At least until some answer says "Yes, the
platform allows to zoom, but only from C#, not from XAML". It's like
asking "Can you programmatically create folders in windows" and
tagging it with "c, delphi, c#, python, script..."
Yes and no. The answer may very well be language agnostic, but the context of the question is not, either from the perspective of the asker or from potentially other viewers.
Using a different example, if someone asked "How can I find the position of 'b' in the string "abcdefg"" and tagged it
c#, string, index, would you remove C#? (You shouldn't.) The fact that something can be done in multiple languages (.NET or otherwise) does not mean the specific language is not useful.
Viewers are looking for language-specific answers. They're searching google for C# string index operations. Removing the language tag is counterproductive towards solving those problems, even if the answer turns out to use the method
IndexOf, a .NET base class library method and not a C# method. It would be useful to add .NET, but removing C# should not be considered.