How can I learn image processing in a practical way? I am a beginner in image processing and I have to learn to program in MATLAB as soon as possible. I started with Rafael Gonzalez book, but understanding this theoretical knowledge and mathematical aspect from one side and bringing this knowledge into practice for doing my project from the other sides, it is a challenging task for me.I am working on 3D medical images. So I should learn how to do pattern recognition with these images.

Do you have any tips on how to translate my theoretical knowledge into practical knowledge? 
Which programming language is suitable and convenient for learning image processing/computer vision and doing practical projects?
The book you mention in your question is a highly practical one. In fact since digital image processing (DIP) is by definition an applied field, any book on DIP must be and will be practical. They should be showing results of algorithms on images. But if you mean actual code, very little number of DIP books would give away anything other than pseudo or matlab / octave code.

In my opinion the preferred language for research in DIP is Matlab / Octave or Python, and the standard tool for production environment is C/C++ with MSVS (licence) or a number of excelent free alternatives.

Note that pattern recognition and medical image processing are two sub-fields of signal processing and image processing respectively. The former subject requires a lot of statistics and probability theory background, which is not included in a typical DIP book.