Change language of captcha

I am using the free version, and using multi-site, where each site is a different language.
But when I go to the WordPress login screen, it shows all the fields in French, except for the “I’m not a robot” checkmark which stays in english.
I also use the shortcode
[bws_google_captcha]
in a Contact 7 form, which is all in French, but it too shows the captcha in english.

I’m using the Multisite Language Switcher plugin.
Any hints or tips on how to change language used based on the site?
I can add some PHP code to functions.php in my theme, or to template files if needed.