How to Transfer Student Visa - Work Visa / Permit ? ( after Diploma)

1- then how much time Govt allow him to stay in Malaysia and find work ?
None - its a student visa and once course is over you must leave

2- Is he/she allowed to find work during their studies ( but will start right after completion of course , like some companies hire student Interns )
Yes, but not during study period (See response to your other posting)

3- What is the Probability of finding a Good Paid salary Job after completing a Course from Malaysia ? ( not specific institute )
Almost zero as not able to employ people under 27 years of age (23 in IT jobs)

4- Process and time taken for transferring Student Visa to Work Visa ?
There is no transfer process. They are handled completely separately.

5- Is it mandatory that student must leave country and come again ( for transfer) on work visa ?  or this could happen without leaving Malaysia ?
Not mandatory on an Employment Pass (Min salary RM5k per month)
Mandatory on foreign worker pass (no minimum salary or age - but mainly unskilled jobs in services sector or in construction).

If you dont attend classes and give up your studies, your student visa will be cancelled and you must leave the country. Immigration work with the educational establishments to control this, some more than others. But it is getting very strict and getting stopped by police in the street to show your passport, happens a lot.

oh Thanks so much u cleared all my queries

Just one more thing  ,
Is it mandatory for international students to take Malay Language course  ?