addons.opera.com detects which language your OS/browser is set to and provides descriptions in that language if available. If you want to change this, you can change the filter language on the search results page.
For example:
https://addons.opera.com/en/search/?query=youtube
There's a box in the top right corner where you can choose a specific language to show results for.
(I get a few search results here, btw)
If the extension is localized in multiple languages, there will be an "Other languages" list on the right hand side on the page for the extension.
