Text mods do not work at all for the Return of Rome expansion. The game completely ignores the key-value-modded-strings-utf8.txt file inside the \modes\Pompeii\resources\es\strings\key-value\ directory.
In contrast, this exact same system works perfectly fine for the normal AoE2 base game.
I’ve verified that the mod doesn’t work locally, but it does work when I upload it to the website. However, after downloading it, the modified description only remains until I restart the game; at that point, the text reverts to normal. This seems to be a pretty serious bug, since upon restarting, the game ignores the mod and loads the official base file directly.
The correct line is: modes\Pompeii\resources\es\strings\key-value\key-value-modded-strings-utf8.txt
In short, after restarting the game after downloading, the system ignores the modified file and prioritizes the official text.