English is sometimes promoted as an easy to learn world language which gives people access to all the academic knowledge in world. All the books, most websites and even movies are created native for an English speaking audience so it makes sense to invest a bit of time in become fluent either in the UK or the US dialect. The problem with learning English is, that the vocabulary is hard to memorize. For example, if a native speaker of French likes to write a letter in English, he needs to lookup a word in the French to English dictionary. For example, he knows that he like to start the sentence with the normal “bonjour”, but what he doesn't know is how to translate this into English.
If somebody is not able to speak the language very well, he doesn't know how to translate a sentence into English. He can ask a bidirectional translater which explains, that bonjour is translated into “good morning” but can the word used in the same way like the french vocabulary? In most cases the answer is no and the French speaker will make his first serious grammar issue and the reader of the letter will recognize, that he is not familiar with the English very well. What he is producing, isn't a letter in fluent English, but his typed in words would read like a beginner translation from French to English.
The better idea is to avoid the foreign language from the beginning and write the entire letter in the native language which is easier to read and will show that the speaker has learned his own language on an expert level. Even if the other side isn't able to understand French he will notice, that the letter was written well and he can ask somebody who can translate it. In the modern age, the translator can be an automatic software.