"El are un socru."

Translation: He has a father-in-law.

[Coriander18]

By mistake I put 'he is a father-in-law' and the correction came back 'he's a father-in-law'.

This would definitely be wrong in Uk English as we don't ever use the apostrophe to change 'he has' into 'he's'.

Do any English speaker use the 's for anything but is?

[cosmo-pedant]

Coriander18 -Your "he's" comment reminds me of the occasional, understandable, but primitive error of saying/writing "would/should/could of" instead of "have.