You search for weapons the same way you would search for any other type of card. You may have misspelled "weapon" or had a class selected that did not contain any weapon cards.
As stated on the Hearthstone Gamepedia page "Collection manager" (emphasis my own):
The function searches not only within card titles, but also within card text, class, type, subtype, rarity and card set. Any card which includes the exact search text in any of these locations will be listed. For example searching for "weapon" will return all weapon cards, as well all cards which include "weapon" in their title or text.
As also stated there, per @mabu's comment, the only thing not different between English and non-English clients is the deck names:
When switching the language of the Hearthstone client, deck names are the only text strings not to change to the selected language, since they are potentially user-created. This includes default deck names ("Custom ", or the name of the Deck Recipe used).