Merge pull request #1954 from ProjectMirador/1876-i18n-locale-toggle

__tests__/integration/mirador/language_switching.test.js

__tests__/src/components/App.test.js

__tests__/src/components/LanguageSettings.test.js

__tests__/src/components/NestedMenu.test.js

__tests__/src/selectors/index.test.js

locales/de/translation.json

locales/en/translation.json

src/components/App.js

src/components/LanguageSettings.js

src/components/NestedMenu.js

src/components/WorkspaceMenu.js

src/config/settings.js

src/containers/App.js

src/containers/LanguageSettings.js

src/i18n.js

src/state/selectors/index.js