The English language offers some very interesting ways to shorten names.
First, it is common to abbreviate double names (two first names, as opposed to double-barrelled names which correspond to two last names) by their initials. For example, the son of Anthony Soprano in the fantastic TV series The Sopranos is usually called A J (instead of Anthony John). There is a crucial difference to German, because while in German you may abbreviate Hans Peter as H P in a written document, I have never heard of anyone named Hans Peter call himself H P (exceptions do exist, as I have just learned from a reader). Other common names in English are T J and C J (if you are in my age group, you may remember C J Parker from Baywatch, and C J Gregg from Westwing). For some reason, the short form seems to be most popular if the second initial is J.
Second, in English, you can abbreviate the surname of people you know (but are not on a first-name basis with) by using only the first letter. Most of the examples I remember involve children who address adults (for example, fathers or mothers of their friends). Hence, Mrs Soprano becomes Mrs S, and Mr Simpson (yes, Homer) becomes Mr S (an example is given here). In the case of English, it seems that such abbreviations were made popular by the sitcom Happy Days, where Mr. and Mrs. Cunningham were called Mr. and Mrs. C. To my knowledge, such a short form is not used at all in German (apart from using initials in order to avoid mentioning real names in news articles, etc.). Do you know if it exists in other languages?
Finally, in English, Mr/Ms X is used for a person whose name is not known.