Basically, the "one you know" is a good start. But...if you want to write an application that works on Android and iOS, then you are pretty much looking at C# via
Xamarin[
^]
Be aware that that is not a free solution - they have a free version, but it's very limited, and the costs can be significant unless you are selling a lot of copies of your app.
Other than that, for Android, you pretty much want to learn Java, and for iOS it's Objective-C. Neither of these are going to take you too long to learn, but the framework behind each will take a considerable amount of time to learn before you are proficient enough to write "good code" for either device.
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