Best Programming languagee to create application

Hello! I am a frrelance programmer. I recently got a project to develop software related to email conversion. However, the application has various adadvanced features that require adadvanced programming. Can anyone tell me the best language I can use to create the application?

Here is the reffered application - https://xtraxtor.com/mbox/pst.html

MY client send me this referral site, they need same program. Please suggest me so that I cam deliver the project at a given time.