Malayalam took the first installed file mapping it: on macOS Arial Unicode MS, which has no Malayalam shaping, so conjuncts like ക്ഷ fell apart. Fonts made for Malayalam are tried first, loaded on demand through the CJK chain lookup.
IDML names some languages by locale code
(AppliedLanguage="$ID/de_DE_2006", "$ID/nl_NL_2005") beside names such
as "English: USA" and "Spanish: Castilian". language_tag maps any locale
code (ll, ll_CC, ll-CC, with a reform year after them) to its BCP 47 tag
(de-DE, nl-NL; kept once per process, at most 512 distinct tags), ignores
IDML's $ID/ prefix, and knows Spanish: Castilian (es-ES) and Swiss French
and Italian. The stored name stays as written, so IDML round trips are
unchanged.
What depends on the language now goes by the tag: typographer's quotes
(by language subtag; Swiss regions take guillemets), English
hyphenation and spelling (is_english), Korean word breaking and the CJK
fallback chains (ja-JP, ko-KR are Japanese and Korean whatever the
region).
Characters the primary font lacks try a fallback chain by language
before the language-blind search: Japanese (the craft-fonts Japanese
faces first, Mincho first), Simplified Chinese (also plain Chinese),
Traditional Chinese and Korean each list their macOS, Windows and Linux
system families. Hangul, kana and Bopomofo pick their language's chain
whatever the character language; ideographs and CJK punctuation follow
it, and keep today's fallback without a CJK language. Chain families
load from the system font catalog the first time they are needed.
designcraft_fonts::testing (feature `testing`) builds synthetic fonts
with a given family name and coverage for tests.
Marco Arnulfo