Recently, I stumbled upon a family photo album. While browsing through it, I found several letters from the early twentieth and late nineteenth century, all written in Dutch. Despite this, I noticed that the use of language, even the use of formulas, is quite similar to that found in some English letters from the same period. For this reason, I think it might be possible that some Dutch letter-writers were influenced by English ones or that there was a universal guide to letter-writing at the time. I also found that there were bilingual letter-writing manuals available in the early twentieth century. This suggests that bilingual letter-writers did exist and more surprisingly, it shows that epistolary formulas and words had equivalents in other languages. Furthermore: for these guides to be published, there must have been quite a few potential buyers. Despite this, I have not yet managed to find any bilingual letter-writers. As I would like to do some more research on the subject, I have some questions:
- Does anyone know of any bilingual letter-writers who lived during the eighteenth, nineteenth, or early twentieth century?
- Were there published any bilingual letter-writing manuals in England in the aforementioned period?
- Was there a universal guide to letter-writing?
I hope someone will be able to answer these questions. Thank you in advance.
Late Modern English letters 
Could you give some examples of the kind of formulas you found?
As a stranger here, and non-linguist, I stumbled on your interesting question and cannot resist commenting that lots of bilingual letters were written by the Dutch upper classes in the 18th, 19th and early 20th century in the sense that they were written in French, but the writers regularly reverted to Dutch when French words were lacking, or seemed too formal. Many members of the upper class were fluent in English as well, and wrote in that language; I have found a friendly letter (1902) by a female writer combining French, Dutch and Italian, with a few German words thrown in. (My apologies if this was not what you meant at all.)