Anbei die Gedanken eines Sfrs aus Grossbritannien. Vor allem die Bemerkungen über die benutzte Sprache sind besonders wertvoll:
Hilda was also a very common English name at that time (but is much less common now). I have no doubt that the person writing this letter must have lived in England for many years, and is certainly very educated. The vocabulary is very wide, with unusual words being used but always in a completely correct way. The language also has several words and phrases that only someone from England would use or know what meant – such as ‘drive one dotty’, meaning to make someone angry and frustrated, or ‘chatty letter’. You may have heard of the slang verb ‘to chat’, being a friendly talk about personal news. To then turn this into a slang adjective ‘chatty’ to describe a friendly, personal letter would mean that someone was extremely familiar with the English language. These are not words or phrases that would be understood by other English speaking nations or peoples, such as South Africans or Americans. The language is of educated people from southern England.
I would suggest that the receiver of the letter is English, and that she herself has written ‘No. 42 / arr. I.IV.45 / last letter from Agram written 28. III. 45’ on the front cover, as this is her natural language.
My guess is that the husband will have been Dutch but educated in England, where he met his future wife and that she followed him into Europe when he chose to work with the Germans. There was quite a pro-fascist following in Britain before WWII, particularly amongst the more wealthy and educated people.
The language of any correspondence being censored had to be marked but I am not sure that it was forbidden. I have got examples of other items which have gone through censorship but are written in ‘enemy’ language. Also, of course, as this was a standard feldpost item travelling within Greater Germany, it would not be examined by anyone.