Factor out language selector into its own component

[miorel]

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[github-actions]

PR Health Report

Last checked commit 2648041.

• npx prettier --write .github .vscode && ! (git status --porcelain | grep .): ✅
• (cd tools && yarn format && ! (git status --porcelain | grep .)): ✅
• (cd tools && yarn lint): ✅
• (cd tools && yarn typecheck): ✅
• (cd tools && yarn test): ✅
• (cd tools/adventure-pack && yarn build-app): ✅