They're both correct, but I prefer "develop a habit". 'develop a habit' as well well as 'creating' a habit, gives the idea of the steps you must take to achieve it. My opinion only, but 'form a habit' is more just about 'creating' that habit and less emphasis on the steps you take to get there.

There is an adjective “habit-forming”. Something which is habit-forming induces a biochemical process to which humans respond, by inducing the release of hormones. So forming a habit is less deliberate than developing a habit. So something that you do because you want to, such as in your example, needs “develop”. The other word to keep in mind is addictive. That’s more extreme than “habit-forming”and would usually be used for drugs. But you can say both “drug habit” and “drug addiction.”

For this case, you would develop a habit of writing an English diary every day. "Form a habit" can have negative meanings associated with it, whereas "develop a habit" implies more of good qualities. You would form a drug habit. You would form a smoking habit. You would form a drinking habit. You would develop a reading habit- You would develop an exercise habit. You would develop a writing habit. The drugs, smoking, and drinking are things that aren't healthy or beneficial. The reading, writing, and exercise are things that are good to do. Developing a habit also implies the process to getting there is important. Forming a habit doesn't place emphasis on the process.