I am submitting these diagnostics under Berta Caceres because this PC running 16.4 is also affected by this language substitution problem. Obviously something I did incorrectly last time I ran apt-get update followed by apt-get dist-upgrade. Only facebook has changed to portugese so far, I have found nothing else affected. My laptop running 17.4.1 is badly affected with english replaced by portugese and Spanish and maybe italian language.
I have to use Spanish keyboard layout but I install with what I believed to be english language downloads, kept up to date every 3 months.
I would like to know what is the cause, how I can prevent it, and how can I reverse it without a new install?
I will shortly submit diagnostics for the laptop problems under Heather Heyer.