Hi thank you for your question at Q&A. I understand that you are running in to 404 HTTP errors when attempting to use Language Detection in cognitive services.
As a first step to using language detection you will need a Language resource which you can create in the Azure Portal. You can use this quick link to create a language resource. There is a free pricing tier you can use.
You need the key and endpoint from this language resource.
Then you can make requests to the new service at https://<your-language-resource-endpoint>/language/:analyze-text?api-version=2022-05-01
In the request you should set a header Ocp-Apim-Subscription-Key:<your-language-resource-key>
with your key for the service.
There is also a good Quickstart guide for REST API and various programming languages on other tabs of the same page. I hope that helps to get you started.