I had the same issue, with an empty additional content window. Based on your steps, I figured it might have more to do with the regional settings rather than the language selection. For example, I had English as the system language, but Finland - Finnish as the region. So I changed the region to United States, and when I started Logic Pro X 10.1, sure enough the additional content window wasn't empty anymore. Hope this also helps.
System 1: iMac (21.5", Late 2009) + Logic Pro X 10.0.7. System 2: MacBook Pro (Retina, 13", Late 2012) + Logic Pro X 10.1. Focusrite Saffire Pro 24 DSP, Steinberg UR22, Yamaha HS50M x2, Alesis Q25, Audio-Technica AT2020 mic + ATH-M40fs headphones, etc.