A variant, script or region subtag never selects the language system HarfBuzz picks outside zh, e.g. el-polyton, ro-MD, yue

[jakejackson1]

OtlTags::language() (src/Shaper/OtlTags.php, upstream Otl::_getOTLLangTag() at src/Otl.php:6150) looks a tag up by its first subtag in Ucdn::$ot_languages. After #201 Chinese is the one exception. HarfBuzz 14.3.1 also maps several multi-subtag tags to a language system the first subtag doesn't give. They come from hb_ot_tags_from_complex_language() and the extended-language rule in hb_ot_tags_from_language():

• Variants and scripts that apply to any language (hb-ot-tag-table.hh:1627-1689): -polyton → PGR, -provenc → PRO, -fonipa → IPPH, -fonnapa → APPH, -geok → KGE, -syre/-syrj/-syrn → SYRE/SYRJ/SYRN.
• Language and script or region: ga-Latg → IRT (:2252), mnw-TH → MONT (:2651), ro-MD → MOL then ROM (:2741).
• Chinese languages other than zh: cmn, yue, nan, hak, wuu, lzh, gan, hsn and a dozen more have no key in Ucdn::$ot_languages. HarfBuzz maps each to ZHS, except yue → ZHH and lzh → ZHT (:517-1141). It also gives each one the same script and region rules as zh (e.g. yue-Hans → ZHS, cmn-Hant-TW → ZHT).
• Extended language subtags (hb-ot-tag.cc, hb_ot_tags_from_language()): where the second subtag is three letters, HarfBuzz looks that up instead, so zh-yue → ZHH and zh-lzh → ZHT.
• More than one tag for a language: the two-letter table lists ga → IRI, IRT and nv → NAV, ATH. HarfBuzz tries each in turn, and Ucdn has only the first.

I measured each case below with a font whose script offers only HarfBuzz's tag, with a locl lookup under it. hb-shape --language selected that tag in every case. mPDF is OtlTags::language($lang, …) with every tag offered:

lang	mPDF	HarfBuzz
el-polyton	ELL	PGR
ga-Latg	IRI	IRT
ro-MD	ROM	MOL
mnw-TH	MON	MONT
oc-provenc	OCI	PRO
syr-Syre	SYR	SYRE
en-fonipa	DFLT	IPPH
ka-Geok	DFLT	KGE
yue, yue-Hant-HK	DFLT	ZHH
cmn-Hans	DFLT	ZHS
lzh	DFLT	ZHT
zh-yue	ZHS	ZHH
zh-lzh	ZHS	ZHT

Expected: follow HarfBuzz for these. #201 adds the ordered-candidates shape (ZHTM then ZHH) that ro-MD and the multi-tag languages need.

Found while fixing #201, which covers only zh.

[jakejackson1]

Once OtlTags::language() handles zh itself (#207), nothing reads Ucdn::$ot_languages' five zh- keys (src/Ucdn.php:778-782), and 'zh-mo' => 'ZHT ' contradicts the ZHTM-then-ZHH behaviour. Whatever fixes this issue should delete those keys, or move all Chinese mappings into one place.