Do I need to study translation in order to work in the field of translation?

Not necessarily. Most translators have not studied translation. Translation was not initially offered as a subject of academic study, and when it was it became available at a limited number of institutions. In the meantime, translators rose among those who enjoyed and appreciated more than one language. These people make the majority of present day professionals. They and others can also benefit from the academic study of translation.

I wouldn't say it is necessary, but it is good to have studied it, so you can show your diploma to potential employers. Plus, you learn translation techniques in your course, which can be very useful in your future career.

Yes it is compulsory because translation has many specializations such as literary, legal, medical, economic etc..so one has to learn these fields in order to translate proficiently. Also, translation requires an expert knowledge of methodology as well as writing and proof-reading skills in order to avoid common language mistakes. Analytical skills are also a must in order to understand the original text correctly and to deliver the most accurate meaning without any comprehension barriers.

If you are a reader, you will be a writer :)

Yes, and after completing the desired degree, one should seek to be signed up in the Translator Association to be addressed as a legal translator. 

first of all, i have to study translation in general,then i must specialize in legal translation

Yes , Of course . With studies , you can improve your translation skills. 

yes i think it is very important if u want to be a provisional translator 

IN MY OPINION YOU IT IS A MUST

I think not necessary to study translation as you have experienced in translation field, but holding a certificate means something significant, otherwise, You can develop your translation skills by practicing translation till you get professionalism which sure leads you to work at translation field reliably.

Yes indeed. To translate a text professionally, you have to study translation or to study literature of the target language. 

If you have a very good knowledge of both: the detect language and the target one, you may translate well but you will NOT be like the one who studied.