Did the term Earth have any other special meaning before humans knew what a planet was? Are there modern languages that do not use a translation of earth for the name of our planet?
Earth comes from the Old English word "eorþe" which meant "ground, soil, dirt, dry land; country, district." That word itself came from the proto Germanic word "ertho."
Other related languages have similar words.
The Earth being called Earth as in planet Earth comes from about 1400.
http://www.etymonline.com/index.php?allowed_in_frame=0&search=earth&searchmode=none
Edit - The þ in eorþe is the letter thorn. It makes the "th" sound.