Search tool in Vocabulary section
Sometimes I need to search for particular words in my vocabulary list, this would be a really helpful tool.
Seconded. Also, it'd help to get the search tool a bit "advanced search"-like: look up for the word in your own language or the target one ("i don't remember how to say dog" vs "What did Hund mean again?"), allow wild card characters (find al the words ending in xxx, or i want to look up this word but am unsure about the spelling), etc. I think it'd be nice to also have the option to look up the entire vocabulary on duolingo, rather than only what you learned so far: easy way to look up for words based on the same root that something you just learned, for example...
Not sure about the need for a 'natural language' search like you're suggesting. I'd be happy to be able to type a word in, either in the language I'm learning or my own language, and see some results.
If you want to look up the entire vocabulary for a language, open a new tab, google 'dictionary' :p Seriously though, don't want the Duolingo staff spending time making a dictionary when they could be implementing other learning tools that aren't already available.