Besides the grammar and spelling, which is easy to correct, English scientific writing has a different style. I often notice that non-native speakers write in a very wordy, verbose style. Also, the organization of the papers tends to seem unclear and unfocused. Their spelling and grammar are perfect, but they don’t use clear topic sentence etc. The paper does not always follow a clear linear progression. This is also part of writing scientific English yet no one sees this as a problem or seems to care about improving this aspect.