I suppose that the tagging UI is quite trivial, you just need an single icon that iterates the languages in loop when pressed. And many users will have just one language, so they do not have to touch anything after the onboarding.
Then using a service you cannot "upgrade" to the region labeling, and you have to manage the incorrect detection issue; finally a service have be manteined and be economically sostenibile. Better to offload this easy task to the users.

 

 I am just trying to get a test out and the only way to test it is to have sufficient tags in all posts with all clients. One client doing this won't work because the user will miss a lot of posts that are not tagged.

A service can test the idea ery quickly without modifying any client. If no one is willing to run the service, then I trully question if this feature is actually needed or if it is just a hollow feature that nobody actually cares. 

 For testing, a service could actually be useful. But filtering notes tagged by a third party should be definitely more complex, right? 

 There are language feeds to test quickly wss://feeds.nostr.band/lang/pt 
 

 Not really. I would do a service that creates 1985 tags with the language code using the same created_at the tagged post has. 

Then a client can just do a REQ for the latest 1985 tags and load all the event ids individually. A webpage could easily do that and I think it would work on Coracle right now, without any changes.  

 > and load all the event ids individually

So two REQ instead of one. I am happy if this is not a burden. 

 These days almost anything in Nostr is 3-4 reqs away...