I'm not sure about your specific issue (level newcoolevel), but I can help you with the Translated String Table.
At the top middle of the Editor is a button that says "TranslatedStringKey". If you open that up, you can create a new file there and see a spreadsheet-like thing that you can edit.
The important columns are the "Key" and "Content" (or whatever was next to Key) columns. The Key column is where you put the names of the Strings that you will refer to elsewhere in the code. The Content column for that Key is what actually displays in-game.
So for your issue, you might want to try putting "newcoolevel" (without the quotes) into the Key part of that, and then "New Cool Level" (without quotes) into the Content column next to it. Then save and see if you get your error again.
The TranslatedStringKeys are what the game refers to when it wants Localization files.
