1. HyperTextAccessibles already cache language and RemoteAccessible::Language uses this.
2. Previously, we didn't cache language for a non-text, non-HyperText Accessible at all. This includes images and HTML radio buttons. Now we cache it as a top level attribute in this case and return it in RemoteAccessible::Language.
3. We previously cached language for a text (leaf) Accessible where the language differed from its parent, but we never used this in RemoteAccessible::Language, only when calculating text attributes. Now we use it in RemoteAccessible::Language as well.
4. Where a text (leaf) Accessible's language is the same as its parent, RemoteAccessible::Language now gets this from the parent.
Differential Revision: https://phabricator.services.mozilla.com/D212503
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