I didn't notice it right away. I didn't mean to do it in the first place. I let my accent slip out of Midwestern English. It happened once, then twice, and now I'm only using Midwestern English when I need to be clearly understood.
So way back in the stone age, I was born and raised in Southeast Missouri. I lived less than 30 minutes from the Arkansas border. I also, for a time, went to college in Arkansas, and did weekend trips to Memphis. You could say my unaltered accent has some natural country thickness to it. Growing up, I chose to learn how to control it. I did this in part because I worked at a call center, where answering the phone in different accents helped alleviate the boredom. Also, because I wanted one of the only 3 Chinese exchange students in town to understand me.
Over the last few months, more and more people have been willing to talk to me in Chinese. I have been really stoked about this. I am finally reaching a level where it isn't frustrating for them to talk to me in Chinese for more than 5 minutes, and I am starting to understand the majority of what they are talking about. But it's not only that, I have over time stopped trying to talk to them in a Midwestern colloquial free accent.
Incident 1So, there I was last month, tired, just got off work, doing a couple of things at once while talking to my Chinese roommate. I toss out a couple of sentences at him without cleaning up my accent. He looks at me like I just spoke Turkish. This happens a couple more times over the next 5 minutes. At that point I stopped and said something like, "我应该用中文呢？" He immediately agreed.
We were doing a big dinner at a friends house. The wife, me, and about 7 other Chinese friends. We were talking about about someone's shenanigans and I guess I was speaking too fast, or using too many colloquials in my English, because one of our friends stopped me with a "I don't understand?". Rather than repeating myself in English I switched over to Chinese, and this caused the whole conversation to continue in Chinese.
My wife's brother has a pretty good command of English. Since I knew him before I knew any Chinese, we usually speak in English. We arrived in Taiwan after 30+ hours of flying, transfers, and minimal sleep. He asked me a few questions and my reply came out full on southern with words drawn out and mashed together. He asked again in Chinese and was happy to get a reply he could understand.
Using this to your advantage...
For the most part my friends just want to be understood. They don't really care what language we use to communicate as long as it isn't frustrating. I bet a lot of people are this way. I'm not suggesting you intentionally obfuscate your speech. I'm suggesting you talk more normal. If they care about their English they will work hard to understand you, and be better for it. Otherwise, if using Chinese is less painful, they will switch to it and you will be better for it.