You are right that it has been available for long time, but you have to consider that it's still "experimental" and thus in better case they dont care much, but in worse case they would find out it does not earn money, or some other reason, and remove it. I rather think they want to implement all the languages and keep what they have been working on - Spanish language has quite big user base and it cannot be useless. Everybody here is asking me whether they have Spanish.
But given that the feature is experimental, it's possible that when they make new versions, it does not matter much if the feature is broken in some versions. It's like officials for the bad people: they try to give you all things, but they do not want to move for this, but they are rather careless. So, if they plan to release something in August, this year, then if more important features have bugs, they would probably move this time further or work harder - but if an experimental feature fails, they are "guilty" themselves, and nobody would slow down or speed them up. So it's really enough if the people of an experimental feature cannot catch up with changes in the application - they simply get nothing.
So if they have created a strong team, integrated it with the process in microsoft, established user connection for testing and improved budgets of this Spanish language, it would keep up with other parts of the company, but when the feature is experimental - they only have luck, no destiny; if they don't have luck, they won't release it. Others have destiny in sense that unless of major force, they would simply be helped through the whole process.
So, you should rather think of it as constantly getting broken in one or another way, if an experimental feature is a part of a bigger project with real budgets and big teams - so maybe you just go to microsoft idea exchange and vote up this idea to finish it faster (https://techcommunity.microsoft.com/blog/microsoft_365blog/submit-your-microsoft-lists-ideas-to-the-microsoft-feedback-portal/3382652). If you share this and get other people to vote as well, maybe they get Spain on the map.