Best language to develop apps

Hello,

My name is Alejandro, I’m a Mechanical engineer and I don’t know absolutely anything about app development. I am a business owner and at the moment I use an app called Tsheets for all my field workers. I’m looking to hire some developers to create a similar app that works across various platforms and that can also be used online on any PC/laptop/mac etc. Can anyone please point me out in the right direction, What is the best language for such task? What should be the minimum requirements of the person hired to do the job? and finally, What do you think its a reasonable time frame to develop and app.

Thank you
Alejandro Moncada