Dren Dran's old Blockland Essentials add-on tried to do this. It worked but it wasn't really optimal.
I don't really think there is an ideal way to do this for Blockland. The most solid thing you could do would be just a bunch of GUI menu replacements that have the text properly translated, but that won't cover anything beyond the default game content. You can try to use a translation dictionary file and replace words with their translations on the fly, but that will make the translation pretty horrible since it won't really follow any of the grammar rules of the target language; not to mention that replacing all strings on the fly isn't a very efficient thing to do.