As I got to know this I started learning german language as it is also spoken in Poland.
Not many people speak German in Poland... far less than English anyway...
Students are given a choice of what languages to learn in grade school/high school - English is by far the most popular with German a distant second. That might've been a good choice in the 80's and 90's before EU accession, but not anymore.
The only way German would be useful is if you are working for a large company that specifically wants to hire a multilingual person i.e. English/German, English/French, etc. Those type of jobs tend to be in customer service front end type roles that pay peanuts.
I have completed my diploma and graduation in mechanical engineering.I am fresher with no experience
I think you're going to have a very hard time finding a job in your field without having any relevant experience. You'll find a job, but I highly doubt it's going to be high paying or within your field. You should've studied IT - programming, coding, system admin, development, system architect, etc. like all the other indians that come to Poland. Although who knows, maybe some company will take a chance with you, but I wouldn't expect a high or even good wage.
I remember seeing a job ad quite recently for a senior system architect paying 30-40k zloty a month. I'd be willing to bet some indian or paki dude is gonna fill that job.