Modding API - BC100Dev/osintgram4j GitHub Wiki
With the Modding API still being integrated into the project, you can make your own client modification for Osintgram4j. Now, this is my own idea, and was not taken from the original Osintgram project. Why this was made in the first place, I have no idea, but I think that it is a cool feature.
Language
For your own mod, you have the only choice into using Java. Alternatively, you can also use C/C++, if you want to make a platform-dependent mod. Just make sure that you are on your own, and that the mod itself is not something suspicious. If needed, always throw into VirusTotal or into your installed AV, letting it scan for a while. Loading the C/C++ library is straightforward, you simply do System.loadLibrary("libraryName");, or you do System.load("/absolute/path/to/your/library.dll");. From within the API, doing an Operating System check is possible.