Pick a topic and read through its Thai words, each with an English translation and part of speech. 2 topics, free, no sign-up. Thai script, with a romanization to sound it out. The app holds the full packs.
Watch Your Language puts Thai words like these in a widget on your iPhone Lock Screen and on your Apple Watch face. You see a handful of them every time you check the time, and the app schedules the repeats so the words you keep forgetting come back more often than the ones you already know.
You can add your own words too — from a photo, a page of notes or a spreadsheet.