Thank you for your question to the Wefugee Community!
I am not familiar with marriages in Belgium, though you may want to check this page of the Belgian government.
For US-citizens who want to marry in Germany, the US embassy is also giving some information on their website. In general, it is not a requirement to live in Germany in order to marry here. I recommend you to contact a "Standesamt" (registry office) in a city of your choice to discuss the details and required documents (which may also vary).
I hope this helps and all the best,