The bar at the top of your screen represents you current standing with the other nations. You can also view the exact percentage by viewing your journal. You need to reach 100% with a single nation for the achievement. The easiest ways to raise your reputation are by completing tasks for a town’s administrator or governor, or destroying pirate convoys. Once you reach 100% the achievement will unlock. This will most likely come naturally as you progress through the Trader campaign.
NOTE: Your reputation will decrease naturally over time so make sure to complete tasks or destroy pirates often to hit 100%.