adopt IETF language tags (BCP 47)

[jwilk]

We should use IETF language tags (BCP 47) instead of ISO 639-2 codes, or non-standard names Tesseract uses.

[jwilk]

Here's a (partial) mapping between Tesseract script names and ISO 15924 script codes:

Tesseract	ISO 15924
Arabic	Arab
Armenian	Armn
Bengali	Beng
Canadian_Aboriginal	Cans
Cherokee	Cher
Cyrillic	Cyrl
Devanagari	Deva
Ethiopic	Ethi
Fraktur	Latf
Georgian	Geor
Greek	Grek
Gujarati	Gujr
Gurmukhi	Guru
HanS	Hans
HanT	Hant
Hangul	Hang
Hebrew	Hebr
Japanese	Jpan
Kannada	Knda
Khmer	Khmr
Lao	Laoo
Latin	Latn
Malayalam	Mlym
Myanmar	Mymr
Oriya	Orya
Sinhala	Sinh
Syriac	Syrc
Tamil	Taml
Telugu	Telu
Thaana	Thaa
Thai	Thai
Tibetan	Tibt

The table doesn't cover:

• vertical writing: HanS_vert, HanT_vert, Hangul_vert, Japanese_vert
• Vietnamese

[jwilk]

Most Tesseract language code are either ISO 639-2 or ISO 693-3 codes, possibly with some non-standard suffixes.

Here's a mapping between Tesseract suffixes and ISO 15924 script codes:

Tesseract	ISO 15924
ara	Arab
cyrl	Cyrl
frak	Latf
latn	Latn
sim	Hans
tra	Hant

It's not clear how to map these:

• vertical writing: chi_sim_vert, chi_tra_vert, jpn_vert, kor_vert
• "old" variants: spa_old, kat_old, ita_old
• equ ("Math / equation detection module")
• osd ("Orientation and script detection module")