Information available online tends to be fragmented, I will try to summarize my experience in a comprehensive way.
First of all, PS3, PS Vita, and PS4 does not region lock any titles released on physical media. Any physical games purchased in any region can be played by any console. Downloadable games can be played cross-region on PS3 and PS4 using the presumably family share functionality. This is not possible on PS Vita because only one account is allowed to be used on one device. You must buy a physical cartridge to play a game from another region.
There are quite a number of PSN regions, but they can generally be divided into Japanese region, Western region, and Asian region. I strongly recommend using a Japanese region account as a primary account, because there are many niche games in there that will never release in the West. This is especially true for PS Vita. Besides a couple launch titles back in 2012, most of the vita games are made by Japanese developers. A lot of RPG and ACT games are making a Western release now, but most ADV or Galgame porting with easy platinum trophies are not likely to be localized. When it comes to PSN+ free games, Asian region is very similar to Western region most of the time, whereas Japanese region can give out some really distinct titles.
The language of a game and how many trophy lists it has is a really complicated matter. It’s totally at the mercy of developers and publishers, so the best we can do is just analyze them case by case.
It is relatively easy in terms of different platforms, either everything is in one list or they are separated. From a trophy hunter’s perspective, games that support cross save are naturally easy stacking platinums. For example, the famous Sound Shape has a trophy list on each platform. After earning one on any of the three, you can then enjoy a platinum popping in 10 seconds on the other two. If cross save is not supported then you’d have to play it again legitimately. Having one list across all platforms is not all that bad. In a game called Super Exploding Zoo, you can boost multiplayer trophies by starting the game on both PS4 and PS Vita, then match with yourself. Earning trophies on any of the two will contribute to the same account.
The most common way to earn multiple platinums on one title is by playing cross-region games. This applies to mostly Japanese games. Titles that released in the West first typically won’t get separate trophy lists. To start with, a Japanese game has a Japanese trophy list, then an American list that is usually combined with the European list. Chinese (Traditional is the default) and Korean lists are sometimes combined to form the Asian list, but the Korean version can stand by its own in some cases. Since PlayStation launched official business in China after the lift of console ban, a few games got Simplified Chinese lists. Actually some games have separate German and Russian lists, but I won’t consider them because of language barrier. Some examples below:
Megadimension Neptunia VII has 4 lists: Japanese, Western, Chinese, Korean.
Final Fantasy XV has 5 lists: Simplified Chinese, American, Japanese, European, Asian
Akiba’s Trip 2 has 9 lists: Japanese, Western, Asian, each on 3 platforms.
These examples are kind of an extreme, since most games are not that complex. I bought a copy of NieR:Automata from an American seller on eBay a couple month ago. By unwrapping I was surprised that the rating is in PEGI instead of ESRB. Afterwards the game started in Japanese which is my console system language. After all it only has one trophy list. From the screenshots above it’s obvious that single Korean versions have a lot less players compared to other versions. Of course it’ s partially because most psnprofile.com users are in the Western region, but we can still see the difference figuratively. I’m a fan of these niche Korean trophies, but unfortunately registering a Korean PSN account requires logging into a government website that verifies essentially a Korean SSN. As a result, the only way to get them is to obtain a physical Korean release.
As for Add-on/DLC’s, their region lock is stricter than games themselves. Basically only Add-on’s from the same region can apply to its corresponding game. I’ve seen people saying Add-on’s in Asian region can apply cross-region, but I haven’t verified this myself.