Some students may take both classes, while others may enter the most advanced class, spend a semester polishing their skills, and go to English 1550. However, in some cases, this may be appropriate when the student needs more work. Whether the first language is Chinese or Spanish, all learners go through the same processes and stages, though first language may affect the length of stay in each stage. Errors are glimpses into interlanguage. They are consistent and stage sensitive.
Mistakes are slips, performance errors.
While transfer is not the whole story, some things are indeed transfered from the first language to the second. Russian and Japanese speakers will have trouble with articles because they do not have articles in their native languages, for example. Robert Kaplan began the field of Contrastive Rhetoric more than thirty years ago. He claimed that different language families had different rhetorical structures. This claim has been accepted in outline, though most people feel students write the way they do because of their schooling, not because of some mysterious Sapir-Whorfian structure in their brain.