English is my native language, whereas I did not even start learning Mandarin until I was about twenty-one years old. So there is no way that Mandarin can mess up my use of English … HA HA HA HA HA.
There are a few ways which Mandarin makes it harder for me to use English correctly. I’ll go over a couple of them.
First of all, I recently have been doing more Chinese -> English translations than … I ever have before. This means that not only am I exposing myself to a lot of Chinese written by native speakers, but I have to pay much more active attention than when I am, say, simply reading a book in Chinese. Ever since this recent spate of translations started, I have found that sometimes the Chinese way of expressing an idea is popping into my head before the English way of something pops into my head, and then I have to translate my thought into English before I speak/write. I had not experienced this much since I left Taiwan, and I find it interesting that it is working on Chinese -> English translations as opposed to other ways I have of using my Chinese language skills which is triggering this.
Second, even though English is my first language for most subjects, there are a few areas where, in a sense, Mandarin is my first language, and English is my second language. Tea is a good example. I barely ever paid attention to tea, let alone drank tea, before I moved to Taiwan, and practically everyone who introduced me to tea and taught me more about it did so using Mandarin. Therefore, I find it much more natural to talk about tea in Mandarin than English. This is why I sometimes talk about ‘red tea’ in English, even though that is the incorrect term. That is also why I tend to talk in Mandarin in tea shops in the United States, even though the people who work in those shops are fluent in English (fortunately, they also tend to understand Mandarin, otherwise speaking to them in Mandarin would cause communication problems).
Since I first got serious about hiking/backpacking/camping while I was living in Taiwan, I also feel that Mandarin is a first language for me there. I also always had a significant level of communication about hiking/camping/backpacking in English, so I was never so far behind in talking about hiking/camping in English as I was with talking about tea. However, when I went on my backpacking trips this year, it felt strange to me that I was talking about it exclusively in English, and not using Mandarin at all (yes, I’ve also done hiking/backpacking/camping in Japan, but I used a surprising amount of Mandarin while I was travelling in Japan, including rural Japan).
That said, English is still by far the language I know best, so when my thoughts appear in Mandarin, translating them into English is generally pretty easy. And I am glad that there are some parts of this world which I got to know in Mandarin before I got to know them in English. Perhaps that is the takeaway for people who are learning a new language – once one has a sufficient level of proficiency, take something you know little about, and explore it using the language you are learning rather than your native language.
The Notes Which Do Not Fit 
This is really cool, especially your conclusion paragraph. I will… keep that in mind. 😉
It is much easier to pull off exploring something new in a non-native language if a) you are in a place where the language you are learning is commonly spoken and b) you are dealing with people who do not speak your native language well (or at all). Trying to set that up in the United States as a native English speaker learning Mandarin would have been … challenging. Of course, if one is doing the exploration strictly by reading and writing, that is logistically much simpler, but I feel that I get a deeper experience by having interactive experiences with spoken language.
This reminds me of how I used French as a kid who used mostly English at home and went to a French immersion school. There were several words I learnt first in French, such as virgulle (comma) or Paque (Easter). And my decoding skills were learnt in French, so for a long time whenever I’d say a word I’d only seen written, I’d use semi-French pronunciation. It led to some confusion because I’d end up with very idiosyncratic pronunciations.
Coincidently, I see this comment just after I have inflicted my terrible French on a Frenchman (fortunately, he speaks English well).