IN or AT: in/at several languages

[Van Khanh]

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.

Thanks
Khanh

[Mister Micawber]

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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).
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Native English teacher at Mister Micawber's