Problem: Sometimes a word is shown in many languages that do not interest a given user (much, at a given time). This applies both to the words shown on a page (both lemmas and translations) and to those in suggested in the various search boxes, and can result in inconvenience and confusion.
Proposed solution:
Interface Allow users to pick the languages they are interested in from a list, and to turn on or off the option to restrict words show on pages and during searches. A more luxurious version would show the user words in other languages as well (which can be helpful), but avoid them cluttering the view. Ideally, this feature would also apply to lists of translations, but I fear that would need a drastic change to the way these work.
Implementation I am not sure how hard this would be, but here are a few ideas. Perhaps searching for pages limited to a category would help, but I fear that is not currently possible. Perhaps the Tabbed languages gadget also uses relevant techniques. If is possible in CSS, that could be used to hide unwanted sections, so that cached pages require no extra processing in the server and no script in clients. Filtering translations might become feasible if they were generated from Wikidata.
Who would benefit: Particularly, anyone using Wiktionary (in a language they know, especially English) to support them while learning a specific other language. More generally, anyone with a strong focus on a few languages, which probably covers most users most of the time (though casual users would probably not use such a feature).
More comments: The Tabbed languages gadget goes a long way towards solving this as far as page display is concerned, but (a) does not help with searching, (b) appears not to work in Firefox on Android.
