As of the new 1.19 snapshots, translations and custom translations in resource packs do not work.

In the pictures you can see the format of the lastest version (1.18.2), and in that version it does work.

Steps to reproduce:

1. Make a resource pack

2. Add your language or custom language

3. Link it in pack.mcmeta (if it is a custom language)

4. Select your resource pack

5. Select your language

6. Run the command with {"translate":"<arguments>"}

Results:

If you execute a command with {"translate":"language.name"} it should show your language name, but it shows the name of the English language.