Here is my question: how do you choose the correct level of politeness in Japanese, when translating foreign material that may not follow Japanese rules? Let's say we have a formal setting, e.g. a shop clerk and a customer. The shop clerk was a former collegue of the customer's mother, so she speaks in a very casual way (let's say the equivalent of plain form), and actually ask "do you mind if I speak like this?" Although she knew his mother, it's still the first time they meet, moreover he is a customer, so I guess in Japanese she would at least use the polite (-masu) form, but I was wondering if that would be a faithful translation and, on the contrary, if her speaking in plain form would be perceived as rude (it's only meant to convey intimacy). What do you think?
It would be natural to first think of the social position and standing of the people involved in the conversations so that it is easier for Japanese readers to imagine the mood and relationship of the characters.
If, for example, there is a homestay family welcoming a foreigner into their home you might pose the family speak politely if they are unfamiliar with the guest or casually if they wanted said guest to feel more at ease. The guest can then, in turn, be framed to speak politely if he/she intends to show good manners or be a rare visitor to the country, or casually if totally unaware of the norms or be familiar with the hosting family.
In other words, it is best to keep the constructs of Japanese society in mind when framing your sentences and choosing the proper form, because you aim for the Japanese readership after all.