The S20 is the export version of the Yuan class I believe. They will be good subs for Thailand.
In their surface fleet, Thailand has a total of nine major surface vessels from China.
Six are Chinese frigates. Four Type 053HT class vessels and two Type 025T, Naresuan class vessels. They displace 2,000 tons and about 3,000 tons respectively. The 053HT were delivered in the early 1990s and have Chinese engines, sensors and weapons. But back then (realizing this was almost 25 years ago), Thailand had some real issues with the vessels and the negative feedback drove China Shipbuilding to improvements. Ultimately Thailand added the two Naresuan class vessels which were improved and larger versions of the Type 053 frigate hull form. These were commisisoned in 1994 and 1995, but Thailand ordered and installed western engines, sensors and weapons on them.
Those two are quite good vessels and are each less than ten years old. Here's how they look:
Clearly a very western style, but also an interesting concept. Chinese hull and structure, western engines, sensors and weapons.
The Thai Navy also has two Offshore Patrol Vessels that were designed in Thailand, but built in China. They are the HTMS Pattani (OPV 511) and the HTMS Narathiwat (OPV 512). The first was delivered to Thailand and commissioned in 2005. The second followed and they displace 1,450 tons and also are fitted with western weapons and sensors. They are vey nice, modern vessels too.
Finally, Thailand also has a replenishment ship, the HTMS Similan, which is a Chinese built Type 908, Fusu class vessel. China has one that was purchased unfinished from Ukraine in 1993, the former Vladimir Peregudov, which in Chinese service is named the Qinghaihu. China then built another one for the Thai Navy, the HTMS Similan which went into service in September 1996.
So, the Thai Navy is used to operating with Chinese equipment. Some of it completely Chinese, others adding western sensors, weapons, and even engines.
Apparently with these subs they are planning on an all Chinese fit.