How would you use foreign words that are pronounced differently from their spelling? For example one of my characters is Swedish but fluent in English, he uses Swedish phrases as affectations (not many - and it's part of his character), one in particular is Tjéna, which is an informal greeting. It sounds, to my English ear, as if it is pronounced Chenna.
When the character uses the word for the first time, I have made sure the meaning is clear through the context in which it is used. To my mind, it will not affect the story if the reader doesn't pronounce the word correctly.
I think the use of the occasional Swedish word, in the right context will help round out the character's voice, without having to rely on trying to write him with an accent.
When he refers to his grandmother, he uses an anglicised version of the Swedish word for his grandmother. When he curses, he uses Swedish curses, but I have made sure that he uses words that are not too dissimilar to English. I'll find out how well this works when it eventually reaches a point when I can let beta readers loose on it.