Investigate purpose of LanguageConfigSchemaTest, possibly remove form submissions

Problem/Motivation

LanguageConfigSchemaTest uses form submissions to change language settings. This is expensive, and I'm not sure it's what the test should be covering.

But if we change those to API calls, then will be checking that saving config results in config being stored.... which doesn't need testing for the specifics of language model.

This needs further investigation.

Steps to reproduce

Proposed resolution

Either:
- remove the test
- document that it's testing the actual form

Remaining tasks

Investigate git history and the issue that created the test.

User interface changes

API changes

Data model changes

Release notes snippet