I think it depends on the language, but specifically for the Spanish version I was initially brainstorming the following idea(s):

1. Separating parts of speech (verbs, nouns, adverbs, etc) into different "word decks" and for each deck you play as a different character (warrior, miner, etc.). This would create some variation to keep the player interested.
2. The words you are trying to guess will be above the various things that character is trying to accomplish (enemies for warrior getting killed - ore that the miner gathers - etc.) 2a. For Spanish, Verbs have multiple conjugations - so there might for example be one Large enemy you kill for the infinitive version (eg: "ser") - and there is a bunch of his minions you must kill in succession afterwards which are the conjugations (eg: "soy). 2b. You have a health bar, and getting answers wrong takes away health. The large enemies (infinitive verbs) for example could do more damage than the conjugations.
3. occasionally, other things that are not spanish words will be sprinkled in that might be little power ups or something that you can use (maybe they can be used for hints or something)
4. You go through levels, gain character experience, etc. - which of course is all superfulous, but might make it feel more like the player is actually making progress - trying to get a high score.
5. As you gain experience, you could upgrade the visual look of the character or unlock things.

--These are just some ideas I was brainstorming, and if I am doing this myself, it will take me a while so it would be a slow and incremental process - but I think this might be one of many possible ways to keep it vocabulary focused, while still making it feel more like a game than learning. I'd of course be open to any ideas though.

The mandarin version already made feels more like a flashcard app, with minimal gamified aspects, but I think if I was to start over on the Spanish one, I would lean more into the game feel - with more visuals and such. I think it would also be nice to add a toggle option to either type out the answer or select it with the mouse. Of course, depending on the time and effort, there are a lot of avenues to take, it just is a matter of actual implementation. Getting artists on board would be nice as well, since we wouldn't have to rely on the Creative Commons sprites and could get a more unified theme. Probably wishful thinking, but I am thinking of best case scenario.

Sorry for the long answer, I realized I ended up rambling on a bit. :D