Simple word translator taking advantage of Wikipedia
Don't use the multiple-arity syntax for defn unless you actually need to:

(defn url-decode
  [text]
  (URLDecoder/decode text "UTF-8"))


Your code could use some line breaks here and there. This is how clojure is usually indented:

(defn translate
  [term src-lg tgt-lg]
    (let [translations (get-translations (fetch-url (get-url src-lg term)))]
      (if (contains?  translations tgt-lg)
        (s/replace-str  "_" " " (url-decode (get translations tgt-lg)))
      "&lt;NOT FOUND&gt;")))


and:

(defn prompt-translate
  []
  (let [src-lg (prompt-read "Source language (en, fr, de ...)")
        tgt-lg (prompt-read "Target language  (en, fr, de ...)")
        term (prompt-read "Term to translate")]
    (println (str "\"" term "\" translated from " src-lg " to " tgt-lg " : " (translate term src-lg tgt-lg)))))


Instead picking out the regexp groups by index, you should use destructuring:

(defn get-translations
  [cnt]
  (apply sorted-map
    (flatten
      (for  [[_ a b] (re-seq #"interwiki-([^\"]+).*wiki\/([^\"]+)\".*/a&gt;" cnt)]
        [a b]))))


... or better yet, I think, would be to do something like this:

(defn get-translations
  [cnt]
  (apply sorted-map
    (mapcat
      rest
      (re-seq #"interwiki-([^\"]+).*wiki\/([^\"]+)\".*/a&gt;" cnt))))