Hearthstone Deck Tracker has two independent language settings: one for the user interface, and one for the cards shown within Hearthstone Deck Tracker.
Change the user interface language
This setting controls the language shown in the user interface of Hearthstone Deck Tracker itself, such as the menus, popups and the in-game overlay, but not Hearthstone cards.
Follow these steps to change the user interface language:
Open the Hearthstone Deck Tracker options by clicking the "options" button in the top right corner
Navigate to Tracker > Appearance
Set the Language dropdown to your preferred language
Change the card language
This setting controls the language of cards and card images that are shown in Hearthstone Deck Tracker, such as on the right side of the main window and cards shown within the in-game overlay. It also modifies some game-specific user interface features such as tribe names shown in the Battlegrounds overlay.
Follow these steps to change the card language:
Open the Hearthstone Deck Tracker options by clicking the "options" button in the top right corner
Navigate to Tracker > General
Set the Primary Card Language dropdown to your preferred language