BCP 47 language tags are case-insensitive (RFC 5646 §2.1.1) but the
locale files mix conventions (pt-br.json vs zh-CN.json). On
case-sensitive filesystems, '--lang PT-BR' or '--lang zh-cn' silently
missed the file, _load_entity_section returned {}, and entity
detection ran in English with no warning.
The cache key in get_entity_patterns was built from raw input, so
('PT-BR',) and ('pt-br',) produced two distinct entries, both wrong.
Add _canonical_lang(lang) that resolves any casing to the on-disk
filename stem via lowercase comparison, and route load_lang,
_load_entity_section, and the cache key through it.
Closes#927
Arnold Wender