It's Estonian. However, it does federate with American instances. This means, that all your content that you post gets stored in American instances too.
Think of it like this: let's say you had an American follower on Instagram that always saved all your posts. You might be able to completely delete your data from Instagram. What this one user does with your data is however, beyond your control.
Here it states that the hoster used is hetzner https://lemm.ee/post/45660045. That being said Hetzner is a german company, so I would expect it to be in the eu.
Lemm.ee servers are not hosted in the US and therefore not subject to US jurisdiction.
As others have pointed out, information on Lemm.ee-your posts, comments, profile info, etc. are both public, and federated with American instances.
What information you consider sensitive is dependent on your circumstances and your threat model, but generally I would avoid posting anything you deem sensitive to lemm.ee, due to the public nature of federated media.