Open-source games are not all listed on one single web-site. But most of these web-sites probably use English, so searching for open-source games probably naturally trains English skills. Most of these games probably also use English, for both the source code, instructions about how to compile and play, and also inside the game, which will probably also train English skills. Messages on forums about these games, and bugs-issues/pull-requests, also use English most of the times.

There is a famous web-browser game that is called "z-type" (search "z-type" in a search engine), where words of natural language are the central element of the game, because it is a "typing-game" with the shape of a shmup-game. It is not open-source, but there are at least 2 open-source equivalents that exist. With these open-source equivalents, the language used can be switched easily, to use Spanish for example.

— "Any thoughts on how open-source games might offer unique language learning opportunities?"

Yes, a game could be specifically designed to learn another foreign language.

For example, mixing the "z-type" concept, with the concept of the game called "excellent-bifurcation" (where the game area is split in 2), for example on the left the player could type English-words, and after each word, the equivalent vocabulary in Spanish on the right area.

And if the mozilla-foundation really able to provide a conversational agent, with an API, it could be possible to include language dialogues in an RPG game (role-playing game).

A conversational agent can also be used as a way to play a "role-playing game" in a pure textual way. Unfortunatly I recieve "personal-attacks" with the conversational agent from open-ai.

Maybe the one from the mozilla-foundation (open-source equivalent) will be an "harssment-free" one ?

At least this is what they claim, but I'm not completly convinced they will really do...

(I was also willing to learn other foreign languages with this conversational-agent, but the inclusions of personal-attacks in the replies made me completly stop.)

yeah... alright then, lets take an old emaple, slenderman, that game was so widly popular that, even non english speakers scrambeled to learnt he words, if not understand that "this word means this"  and it was crucial as well to learn these words for the game, the urgney was the fandom and the horror. other games that include this as well are weird games, games like earthbound whic surprisingly has a high forign language community, i recall starman.net having such a community for a while, and... of course, if i mention earthbound, i must mention, undertale... yeah. Undertale was a force of nature, translated in many-many languages and still going strong with deltarune. sometimes its nott the mainstream games that aid, most of the time its the AA or indie games that draw the appeals, or not even those games, but the tradtional learner games, liek freddi fish, bugs the knowledge bug, spy fox, putt-putt, all those humgonus entertainment games which were a cornerstone of american child development. The trick is, "oh look, this word or phrase, its important because it unlocks something, and once you understand your rewarded twice" imagine a casino were if you learned you wer rewarded, yeah id be pretty addicting, but after a fw hours, full senteencs would begin to form.