If he used an interprete to talk, he did not speak enough English to understand everything written or spoken around him.
If it is your first time in UK (or USA for that matter), and you don't understand the language, .. would the airline have had to tell him that when he is in UK he can't leave his baggage not even for 30 seconds, in his language? If you travel inside EU, there is no law to force you to buy a turist guide where they might explain things like that. IF you understand English, you hear all the time in the airports, train and coach stations (in UK) to not leave your baggage unattended. UK has MANY people, it makes sense. IT is not possible to make the announcements in EVERY Euro and other language. So - when visiting UK (or USA) it is your responsability to know English? And similarly when we go to countries where they speak Hindi, Urdu, Finnish, Icelandic etc, it is our fault as we can't udnerstand what we hear?
In Italy ALL the train etc announcements are only in Italian. The only exemption is airports (English too). UK, at least Birmingham is all-English only. So - if you are visiting Italy and take a train - there is clearly written in Italian that you need to stamp your ticket before you get to train. If anyone fails to do so, they get a ticket. Even when the ticket would have enough space to have the infos printed in English, Spanish and French, if you just don't understand the local language, your fault - get fined.

(unless the ticket controllor looses his minds as you fail to communicate in any language..)