A translator/interpreter should be a really good psychologist, should be able to predict or detect any Red Flags, should be able to encourage or comfort, to clarify any cultural or meaning misunderstandings, should not be afraid to do it not once if need arises till 2 parties understand each other to the end.
An inexperienced translator sometimes chooses incorrect expressions that can change the meaning of your words completely, or make your letters sound stilted and clumsy; the last thing you want when you are building up a delicate relationship with a lady. In situations like this, it is as important HOW you say something as WHAT you say.
Relevant experience is essential. It is very good if a person actually lives in the USA, UK or Canada, to really understand all the cultural realities men live in; be able to explain the differences between the two countries.
If a translator came through the fiancée visa process herself and knows all the details, how to avoid some difficulties, save time and how and where to get necessary documents, then she can share all this information with the lady when the time comes, and give advice on that, based on personal experience.