IN or AT: in/at several languages

Please choose “in” or “at”. What is the difference between them in these sentences or the same meaning?

a/ He is proficient in/at these applications.
b/ He is proficient in/at several languages.
c/ he is proficient in/at his job.


They have the same meanings. I would choose at, but that is just a personal choice (unless I have been influenced by my upbringing, education and experience).