Compare prices between PSN Stores
Browse the game list or use the search function. When interested in a game just click the "Prices" button. You'll see something like this:
There you can see that you can buy the SAME game from different stores.
The Content ID is a code that identifies every game. In the previous screenshot the Content ID EP1018-CUSA02010_00-DYINGLIGHTGAME00-E001 identifies the european PS4 version of Dying Light.
If a Content ID appears on different stores, that means that those stores are selling the SAME game version. That means that the languages available and all DLCs will be the SAME.
Please note that this is on a game by game basis. Some games will have different Content IDs on different stores. So for example if a Content ID is unique to the russian store, that means that Content ID identifies a very specific version of the game (maybe only in russian language)
First, create an account on the foreign store. Second, you'll have to add funds to the account (usually purchasing PSN store cards for THAT store).