stalign fails with `RangeError: Invalid currency code : null` in slugify/getCurrencySymbols

One audiobook out of ~50 fails to align, both with the CLI and in the Docker container. The only flag I passed was the convert-to-EPUB3 one. The log ends with RangeError: Invalid currency code : null at new NumberFormat (<anonymous>) followed by a stack through align/dist/align/slugify.js (getCurrencySymbols -> createReplacers -> slugify) and Aligner.alignBook, then Alignment failed — keeping original files.

Resolution: The alignment step derives the locale from the EPUB's language metadata and passes it to Intl.NumberFormat while building the slugifier's currency replacers; if that metadata is missing or malformed, the locale comes through as null and Intl throws Invalid currency code : null. The EPUB spec requires the language field, but real-world EPUBs are frequently built badly, so this shows up on isolated books even when every other book in the library aligns fine. Work around it by passing an explicit locale to the CLI, e.g. --language en-US; the flag acts as an override for the EPUB metadata, so it is safe to set it permanently as your default. Alternatively, fix the dc:language value in the EPUB itself before importing.