What to write the program?

I want to ask desktop programmers for advice ...
I have long wanted to learn another programming language, and here the reason appeared ...

So, the goal: preferably in a short time to write a small & fairly simple program with a GUI and, most importantly, at the same time learn a new programming language

Initial data:
1) There is a solid programming experience in PHP, Javascript, XSL
2) I myself sit under Linux, there is windows in virtualke. I would like to develop in Eclipse under Linux
3) Desktop and GUI applications never developed.
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Wishes (read - requirements) to the finished program:
1) Cross-platform (but main focus on Linux)
2) In an interpreted language (80% tend to Python, 20% to Ruby, I don’t want Java)
3) For Windows, you can create an .exe installer. The smaller its size, the better. After installation immediately works out of the box.

Now questions:
Is it possible to fulfill these requirements? (I know what's real)))
What programming language would you recommend (Python, Ruby, Java ... maybe something else)?
What GUI library should I use (Qt, GTK, WxWidgets .....)?
How to start learning? on what links to resemble? What books to read?

Thanks in advance for the answers-tips!
UPD1: This program is for personal use ... To work with a small XML database using some physical and mathematical laws and transformations. It is possible to build on the basis of the results of simple graphs. For the department of my student's department, I would like to give a gift to a supervisor before a diploma.
The main goal though - to LEARN to make desktop programs with the listed characteristics. Just in the framework of personal and professional development. To begin with, a simple program can be made, then gradually it can be more interesting.