Glossary entry (derived from question below)
Spanish term
Switzerland
Sworn Translator and Interpreter
Native in: Spanish 
Works in: English to Spanish, Spanish to English, French to Spanish
Dn.
¡Muchas gracias!
| 4 +4 | Mr/Mrs/Ms |
Helena Chavarria
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Non-PRO (1): Neil Ashby
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Proposed translations
Spain
Striving for excellence
Native in: English (Variant: British) 
Works in: Spanish to English, Catalan to English
Mr/Mrs/Ms
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Note added at 22 mins (2013-08-22 13:39:40 GMT)
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En Hispanoamérica
En el continente americano el tratamiento se suele otorgar con mayor facilidad. Muy frecuentemente se trata de don a toda persona después de la mayoría de edad o del matrimonio. En ciertos países es el tratamiento dado sólo a individuos que se han ganado el respeto de la comunidad (caso de México). El término Don se utiliza aun antecediendo el nombre, Don Juan o Don Juan Martinez pero nunca el apellido; Don Martinez. en este caso se utiliza el término Sr. pero de igual forma el término señor no se debe de utilizar para el nombre, únicamente para apellidos es incorrecto el decir Sr Juan.
http://es.wikipedia.org/wiki/Don_(tratamiento)#En_Hispanoam....
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Note added at 41 mins (2013-08-22 13:58:28 GMT)
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In the following references, 'Dn' is used in one and 'D' in the other
1948: Capitán Victorio y Teniente de Navio D. Horacio Repetto Peláez.
http://www.histarmar.com.ar/Armada Argentina/Submarinos Arge...
CEX 51-52 Remolcador de Mar ARA "Charrúa"
Teniente de Navío Dn. Horacio Repetto Pelaez
4687 89 - - - -
Buque Salvamento en Isla Decepción. Operación "Progreso" (Apoyo al 1er Vuelo de la FAA sobre el Pasaje Drake - Lat: 60º)
http://www.irizar.org/armada-antartida-39-54.html
I suppose 'Dn' must be an abbreviation which was more commonly used 50 years ago.
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Note added at 45 mins (2013-08-22 14:02:25 GMT)
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Don = D.
http://www.rae.es/dpd/apendices/apendice2.html
| agree |
Cristina Gonzalez
41 mins
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Thank you, Cristina!
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| agree |
Yvonne Becker
1 hr
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Thank you, Yvonne!
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| agree |
Alistair Ian Spearing Ortiz
4 hrs
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Thank you, Alistair!
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| agree |
AllegroTrans
11 hrs
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Thank you very much!
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Discussion
When I said "it's still found, though less often than it used to be", I meant the abbreviation "Dn.", not the use of "don" itself. The latter is still as widespread as ever, but the standard abbreviation for it is now "D." "Dn. was still standard in the nineteenth century but declined somewhat in the twentieth, at least in Spain. Similarly, the female equivalent, "doña", used to be abbreviated "Dña.", and sometimes still is, but the standard abbreviation is "D.ª":
http://www.rae.es/dpd/apendices/apendice2.html
The comparison with "esquire" is very apt, as the use of "don" used to denote that the person was an "hidalgo", which is quite often translated as "squire" in English.
Dictionary of the real Academia Española
I suppose the use of 'Don/Doña' in Spain is following the steps of Esquire/Esq. in English, which I haven't seen for about 40 years!
I see "Mr" in translated documents sometimes when someone has felt that "Don" ought to be translated. To me it sticks out like the proverbial sore thumb.