It is difficult for one to eliminate one's memory of the rules of pronunciation in one's own language when one is speaking English (sentence illustrative of one problem) . Our Canadian prime minister is French, so the local newspaper (very anti the minister's party) always mimics his mispronunciation in cartoons and editorials. That becomes dat (the h in th is silent in French), these is dese and father becomes fodder. I had a boss from Colombia who, after university in the U. S. A. and 20 years employment with a major company, always started words like special with the syllable es- (special is especial in Spanish). I had a Mexican roommate in college who couldn't pronounce bottle (or was he just cagy when it was his turn to buy?). But I also note how quickly a foreigner picks up enough to get by, and sometimes a lot more than someone born here but poorly educated.