When in doubt, translate to the best you can and if it is not gramatically correct append the phrase "[sic]" (everything in the quotes, including the brackets) which is short hand for copied faithfully, mistakes and all. If you feel that the error is such that it renders the quote out of context, you can then use "[sic: your correction]" so that the actual thought is conveyed if there was context to assume the mistake was unintentional. Since you're doing a full letter translation, using [sic] alone should be just fine, as the reader has the entire context of the quote.
For ambiguous pronoun problems, where a pronoun could refer to two or more people. Consider:
"Jim whispered something to Bob. He looked concerned and worried."
In normal English, pronouns typically refer to the last person mentioned, so this here "He" refers to "Bob". However, if a witness is recounting the thing and Jim was the subject of the last sentence, then a writer could simply say "He [Jim] looked concerned and worried."
Again, these should only be used in cases where you are quoting or transcribing someone else's words. Since your case is translating a letter, the words you write in English are not your word's but your original author's, so you need to preserve what he said to the best of your ability. Any amendments to grammer correction or tense would be your own and thus, you need to inform your reader that these are your changes for clarity, and not the original author's own words.
Of course, this is all in a non-fictional context (the letter is written by a real person and not a character). In which case, there are a number of things you can do.
For simplicity's sake, do not show the letter to the reader, but have the characters comment on the language issue and the translator explaining it in dialog (this gives you a bit more room as the translator is also under dialog rules, which means you don't need to use perfect grammar or syntax and can give it in a summation or correct the errors). Given that at the end of the day, everything here are your own words, you're free to take liberties as needed.
Also in fiction, when two people are speaking in a language that is not the Novel's original language, if the POV can understand it, I prefer to use the following
"" Ivan asked in Russian.
Here, the <>, if used consistently, denote that the included is said in Russian and accurately translated for the reader's benefit, who probably doesn't speak it.
Even if you do speak perfect Russian, if your work is intended for English readers, the assumption is that they don't. A long dialog of the Russians speaking with Russian words will be lost on your reader (and be boring). You also run into problems because there are different alphabets that don't work well together. You either have to Romanticize the Russian words OR learn a bunch of character codes to get the Russian Alphabet to show up in your word processor correctly (heaven help you if you then Copy paste that to an application that doesn't understand that character encoding at all).
If the story lies on the POV not understanding what is being said, than just note the conversation is in Russian and use actual language sparingly... either the sentence is important and is included for the readers who do speak the language (called a bi-lingual bonus on Tvtropes) and either is an amusing gag OR plot critical and potentially for shadowing an upcoming event (those nerdy enough to want the clue can run it in Google Translate and get the hint. Those who are not will be in for a surprise.
As a final note, and this is for something where there is spoken language, I would also refer you to the British TV series "'Allo 'Allo" which is about a group of French Resistance members in Nazi Occupied France. To get around language barriers given the writing features native French, English, and German speakers reguarly, the series used comedic accents to denote language use. All the French speakers spoke English in French Accents. All the German characters used English in German Accents. All the English Characters used British Accents. When a character was bi-lingual, changing an accent denoted changing a language. One such character, an undercover British Spy working with the resistance was given the character trait of being a terrible French speaker by using malapropism in his dialog when in French Accent. His catchphrase "Good Moaning" was never "translated" if he was using his real accent, because he could say "Good Morning" properly in English accents. The other gag was one German character who always saluted with the word "plop" instead of the Nazi Salute, not because of any language thing, but because the actor was a real life victim of Nazi crimes and couldn't do the salute even in the service of comedy. Of course, his substitution of "plop" was just further mockery of the Nazis from the show's standpoint and never a translation issue.
I would suggest this method only for performance dialog only (stage, tv, movies, i.e.) as writing accents can be difficult as well.
When writing fictional characters speaking the native POV language but with an accent, make spelling and grammar changes only if the accent is so thick it actually makes the characters difficult for the characters to understand in universe.
In non-fiction, be as faithful to the original speaker's words as possible and not when you are correcting the spelling or grammar mistakes for the sake of clarity.
In fiction, translate in ways that best support the story, but use consistency.
- Avoid writing in accents accept for dialog and only when such accent is critical (such as being near impossible to understand by native speakers). Also, don't forget vernacular First Person (Such as Huckleberry Fin and Tom Sawyer).
As one final note, the Daily Show recently did a story about Diplomatic Translators and how they can sometimes have a hard time translating figures of speech. At issue was many of Donald Trumps phrases (Trumpisms) which aren't conveyed. One such focus was the Japanese Translator for Trump, who pointed out that the Japanese don't really understand the crassness of Trump's Dateline Tapes because, while they understand what part of the body Trump was referring too, they don't really have slang for that part at all (let alone understand all the feline puns and gags) so while it is crass to them, the exact level is lost in translation. They also highlighted that since most diplomatic translators are trained to be diplomatic, some of Trump's more blunt statements are softened when translated into the foreign diplomat's native tongue.
As a general rule in diplomacy, even if a leader is fluent in a language of another dignitary, they will still have a translator on hand because the translator is more likely to be versed in local phrase-ology and can better translate the nuance. Consider Admiral Yamamoto's infamous "Dictate Terms from the White House" claim when he was discussing the Japanese war effort in WWII. Yamamoto was using phrasing to say that winning this war was impossible because there would be so much resistance in what was needed to actually win. What got translated to Americans was that this was his plan. In effect his "We cannot fight this war to completion" was poorly translated to "We will fight this war to completion" by a bad language understanding from non-native speakers.