What programming language to learn for this particular project?

I have a vast batch script cluster that monitors my downloads directory. When it ‘detects’ a completed download, it processes it (using various external command line tools), then moves it to a directory where similarly named files reside. Other functions include duplicate media finder which can use combination of partial filename, media duration and size to find them quickly. The cluster consists of around 20 different functions, big and small. I have much more ideas to expand this into a proper offline media center with extensive automated management features.

I have a dire need to evolve this into a graphical user interface, first of all, and I am faced with the problem of choosing a programming language for it. I do not know any other languages but am willing to learn a new one just for this passion project alone. What language would be THE programming language of choice for something like this, for someone like me, and why? Keep in mind I have never used programming tools other than Notepad++.