Kidney

Werdhom word conservation score = ★★★

English: a kidney, the kidney

Dutch: een nier, de nier 

German: eine Niere, die Niere

Danish: en nyre, nyren

Norwegian: en nyre, nyren

Swedish: en njure, njuren
Icelandic: nýru, nýru

Proto-Germanic: neurô

English: renal (involving the kidneys)
French: un rein, le rein

Spanish: un riñón, el riñón

Portuguese: um rim, o rim

Italian: un rene, il rene

Latin: rēn, rēnis

English: nephrology (the study of kidneys)

Ancient Greek:  νεφρός (nephrós)

Note: the underlined languages have been reconstructed by academics, as there is no archaeological record of the language at that time.