When there is a marginal change, diminishing return or a state of little or no change despite our effort, we would use "plateau". If you picture a plateau in your mind, it would be a relatively flat terrain albeit at a higher altitude as compared to the surroundings. In order to reach a plateau, you have to ascend in altitude first. Therefore, if you use "plateau" in your sentence, it means that you were previously making good progress on your English but you seem to be stuck at a certain level despite your continuous effort in your learning. A "bottleneck" focus on something that stands in the way of progress. The neck of a bottle is narrow and forces the liquid to flow slower. Therefore, if you use "bottleneck" in your sentence, it means that there is a situation which interferes and limits your learning. Some external factors are the causes of your "bottleneck" and once you eliminate them, the traffic resumes at a healthy speed, similar to how cars speed up after crawling past a stretch of road work. This is a nuance between "plateau" and "bottleneck". Is there something which prevents you from your learning, such as heavy work commitments or family errands? And you know that if you are able to remove these distractions and reserve some uninterrupted time for your learning, you will make good progress again? If so, "bottleneck" would be a suitable choice. Do you experience a slowdown in your progress which many language learners naturally do after a while? Perhaps you are now trying to grapple with more complicated grammar rules, structures and native expressions? And all these happen despite your consistent effort? If so, "plateau" may be more suitable. Eg: 1) Population growth has plateaued (verb); in fact, the city is expected to face a decline in its suburban population. 2) He isn't aware that he is the bottleneck (noun) of all these project delays. 3) Production has been bottlenecked (verb) by the lack of raw materials and strikes.

Sure, you could. The detailed meaning wwould be slightly different, but teh end result would be the same. I feel as if my level of English has plateaued. I feel as if my level of English has reached a bottleneck. I feel as if my level of English has bottlenecked. . A bottleneck is a narrowing, an obstruction, a constriction, impeding progress, so the result is very similar. "Plateaued" has to be more common, for a skill. . He works tirelessly, but he seems to have reached a bottleneck limiting progress. He works tirelessly, but actual results seem to have plateaued. . Sales have plateaued. We have plenty of contacvts. Our botleneck is in converting contacts to sales.