Anki (flatpak; some screens require scaling down):
An e-flashcard application, useful for studying any subject, and popular for language study.
(Not the best choice of font colors, and apparently not changeable at present.)
Import your own file, download from numerous available specialty websites; use single words, whole sentences, or any type of study material. Also supports media (images, audio, etc.).
Page through each card in a deck, respond, edit, etc.:
Edit a flashcard:
More info about add-ons.
Generate your own lists of words with this script.
EDIT: Besides vocabulary lists of single foreign words, studying entire sentences of dialog from natural conversation is a great method of building language skills. Foreign films are a great source of spoken dialog, and it’s also possible to download files of subtitles from, e.g. opensubtitles[.]org. Once downloaded and opened with a text editor, the whole sentences can be imported into Anki. But since the subtitle file is full of non-text entries like time-stamps, etc., it’s helpful to extract only the lines that contain the text. See this script for accomplishing that.