there's a glitch when I click "English" for the language, for some reason, it only shows me a handful of cards, and they're in my collection. even though I didn't change the filter to just show cards in my collection
Well apparently you may have done it for a handful:) you bc an go to the specific cards and check their page. In the bottom right menu(…) you can click on “specifics” to set or reset the language
You have also an option on the settings to set the default language when you add a card. If you set it to English all new added card to your collection will be flagged as english
To your question there is no way (yet but I will add that idea to my todo) to filter by foreign cards
The English flag is used to search cards in your collection that you have flagged as English so it seems to work based on your description