MST error in the Classification model tests

[eatyourgreens]

Package

• lib-classifier

Describe the bug

[mobx] Encountered an uncaught exception that was thrown by a reaction or observer component, in: 
'Reaction[_onLocaleChange]' Error: [mobx-state-tree] Error while converting `undefined` to `string`:

    value `undefined` is not assignable to type: `string` (Value is not a string).

To Reproduce

Run the Classification tests in the classifier. locale is undefined, which isn't an allowed value for metadata.userLanguage.

Expected behavior

The tests are passing, so this is minor, but mock data shouldn't throw errors.