Document --lang in the README usage section #82

The --lang option now exists (en default, es, fr; unknown values are an error listing the supported languages). In README.md, under ## Usage, add a row --lang LANG to the arguments table (supported values en, es, fr; defaults to en) and add example blocks in the same style as the existing ones, e.g. cargo run -- --lang es Ana -> ¡Hola, Ana! and cargo run -- --lang fr Luc -> Bonjour, Luc !. Optionally add a short Troubleshooting entry for the unknown-language error (exit status 2, stderr message naming en, es, fr). Verify the examples by running them. Only edit README.md.

Definition of done

• cargo run -- --lang es Ana passes.
• cargo run -- --lang fr Luc passes.
• grep -q -- '--lang' README.md passes.