That's not trivial, that's appalling by the passport bods.
Apparently, Younger Son now has his German passport back, thank goodness.
I asked online about what was an acceptable certificate for translation. The online person gave me a link for "Translating Documents" which said they had a list of approved translators for each country and gave a drop-down menu for which country I was in.
I clicked "United Kingdom".
This directed me to a page that said:
"Unfortunately, we do not have a list of approved translators for the United Kingdom. If you contact the United Kingdom Consulate for the United Kingdom, they may be able to help".
I cut and pasted this on the "online "help"" and the response was "We don't provide a list of approved translators. You will have to contact the Home Office. Here is their Website."
I went to that, and was directed, you guessed it, back to the UK Passports Website...
Eventually, they admitted that yes, the International Translator Certification would be accepted for official documents.
This is pretty normal for the UKPA: one time I carefully placed the countersigned photograph in an envelope, and glued the envelope onto an A4 sheet, with "Countersigned picture in here" in large letters.
They then called and told me off for "forgetting" to send the countersigned photo. I told them I'd send a replacement for the one they lost, and they were quite adamant that no, they hadn't lost it, I'd forgotten to send it...