The problem is the long time frame it will take for the Chinese to establish a submarine arm. For every sub crewman you'll need one more for the shore establishment for training. You'll need yards capable of doing repair and maintenance work of a very high quality, armorers who can maintain torpedoes, etc. All of this is starting from square zero where you have no crews trained, no maintenance people trained, and on and on. It will take 5-6 years to adequately train an officer from junior officer on a sub to commanding officer - yes folks got promoted faster in WWII but that was exceptional. How many years experience did the COB (chief of the boat) have in western navies, probably at least 10. Even if you magically had the boats, China in the 1920s and 1930s had almost no navy, and while the Chinese are certainly as POTENTIALLY capable as anyone else, the educational level in China for both potential enlisted and officers was limited - a small pool to choose from.
For China to build a submarine force of a dozen coastal submarines, you need to build the shore establishment at the same time. You'll probably need one base for the South China Sea and one for China's east coast - one of which is the major training and maintenance. A coastal submarine will have a crew of about 40 (looking at similar submarines in the IJN). So for a dozen submarines you'll need ~960 sub qualified personnel of whom perhaps 50-60 are officers. Some of these will be part of the technical shore establishment and some part of the training establishment at any one time. I can't imagine you'll need any less than 1,000 enlisted and 50 officers to run the base(s), manage repairs and shore based maintenance, etc. All of these roughly 2,000 people need to be highly trained and competent. I'm not counting the shore establishment that manages base services (logistics, messing, barracks - everything that makes a base run). Getting the non-technical personnel for base services is relatively easy.
If you start from zero, as China would have to, I would estimate it would take a minimum of 10 years to have an effective submarine force of 12 coastal submarines with all of the supporting facilities. This is assuming you don't have active military operations going on where the IJA is trying to overrun your base, the IJN or IJAAF bombing your facilities. This project will also be quite expensive, and require significant expenditures in hard currency to buy equipment, boats, torpedoes, trainers, salaries for foreign instructors. Naturally on top of the actual expenditures, you have the levels of corruption that existed in China at this time which will suction off funds raising the cost. China has neither the money, personnel, nor time to do all this.
Realistically China would need to start doing this in the mid-1920s, and even then they would be dependent on foreign sources for submarines, most spare parts, and torpedoes. China simply does not have the technical/industrial base to build submarines and torpedoes in the 1920s and 1930s. As an example look at the difficulties China had to build an indigenous air force where getting adequate personnel for pilot training and mechanic training was a struggle and you had Chinese being trained in the US for these roles.