Do GTA saves on PS2 work across different language version of the game?

TL;DR: If the main.scm file is the same across discs, you can share the save game between those versions.

It depends, let's take Grand Theft Auto San Andreas as an example.

The following documented versions have been released on the market:

• 1.00 JP (SLPM-55292) (Rockstar Classics)
• 1.00 DE (SLES-52927) (Original)
• 1.03 JP (SLPM-65984) (Original, Best Price)
• 1.03 US (SLUS-20946) (Original)
• 1.03 AU (SLES-52541) (Original)
• 2.01 AU (SLES-52541-ANZ) (Original, Platinum)
• 2.01 DE (SLES-52927-P) (Platinum)
• 3.00 US (SLUS-20946GH) (Greatest Hits, Second Edition, The Trilogy)

The 1.00 and 1.03 versions are usually the most compatible with each other. I managed to take a 1.03 US save game and put it on a 1.03 AU disc back on the day (as the 1.03 AU version is not censored).

The jumps to 2.01 and 3.00 are the ones that are not compatible. 2.01 for PAL and 3.00 for NTSC was the No More Hot Coffee patch (if I'm not wrong).

There are still other versions that are not documented, like some discs mastered on 2014 with cut songs due to expiring licenses that are also incompatible.

As a final note, the RenderWare based GTA games (3D/3rd generation) break compatibility with save games with simple changes on the game code (called main.scm).