Add language dropdown to compose in web UI (#18420)

app/javascript/mastodon/actions/compose.js

app/javascript/mastodon/actions/languages.js

...ript/mastodon/features/compose/components/compose_form.js

...mastodon/features/compose/components/language_dropdown.js

.../mastodon/features/compose/components/text_icon_button.js

...eatures/compose/containers/language_dropdown_container.js

app/javascript/mastodon/initial_state.js

app/javascript/mastodon/locales/defaultMessages.json

app/javascript/mastodon/locales/en.json

app/javascript/mastodon/reducers/compose.js

app/javascript/mastodon/reducers/settings.js

app/javascript/styles/mastodon/components.scss

app/serializers/initial_state_serializer.rb

package.json

yarn.lock