Trying to answer myself with some cleaning. So, which existing or
potential Wikisources lack OCR with FineReader (and Tesseract?) but are
interested in Google's, beyond Bengali?
== Languages supported by FineReader, not Google ==
Abkhaz
Adyghe
Agul
Altaic
Avar
Blackfoot
Bugotu
Buryat
Chamorro
Chukchee
Corsican
Crow
Dargwa
Dungan
Dutch (Belgium)
Eskimo (Cyrillic, Latin)
Even
Evenki
Frisian
Friulian
Gagauz
German (Luxemburg)
German (old spelling)
Hani
Ido
Ingush
Interlingua
Jingpo
Kabardian
Kalmyk
Karachay-balkar
Kasub
Kawa
Khakass
Khanty
Korean (Hangul)
Koryak
Kpelle
Kumyk
Kurdish
Lak
Lezgi
Luba
Malinke
Mansi
Mari
Maya
Miao
Moldavian
Mordvin
Nenets
Nivkh
Nogay
Norwegian (Nynorsk)
Occidental
Ojibway
Ossetian
Provencal
Rhaeto-romanic
Rwanda
Sami (Lappish)
Selkup
Somali
Sorbian
Sotho
Sunda
Tabasaran
Tagalog
Tok Pisin
Tun
Turkmen (Latin)
Tuvinian
Udmurt
Uighur (Cyrillic, Latin)
Ukrainian
Yakut
== Languages supported by Google, not FineReader ==
Acehnese
Acholi
Adangme
Akan
Algonquinian
Amharic
Ancient Greek
Araucanian/Mapuche
Assamese
Asturian
Athabaskan
Balinese
Bambara
Bantu
Batak
Bengali
Bikol
Bislama
Bosnian
Burmese
Cherokee
Chinese (Mandarin; Hong Kong)
Choctaw
Cree
Creek
Dhivehi
Duala
Dzonkha
Efik
Ewe
Filipino
Fon
Fulah
Ga
Gayo
Georgian
Gilbertese
Gothic
Gujarati
Haitian Creole
Herero
Hiligaynon
Hindi
Iban
Igbo
Iloko
Javanese
Kabyle
Kachin
Kalaallisut
Kamba
Kannada
Kanuri
Khasi
Khmer
Kinyarwanda
Komi
Kosraean
Kuanyama
Lao
Lingala
Low German
Lozi
Luba-Katanga
Luo
Madurese
Malayalam
Mandingo
Manx
Marathi
Marshallese
Mende
Middle English
Middle High German
Mongo
Navajo
Ndonga
Nepali
Niuean
Northern Sotho
North Ndebele
Nyankole
Nyasa Tonga
Nzima
Ojibwa
Old English
Old French
Old High German
Old Norse
Old Provencal
Oriya
Ossetic
Pampanga
Pangasinan
Pashto
Persian
Punjabi (Gurmukhi)
Romansh
Sakha
Sango
Sanskrit
Scots
Sinhala
Songhai
Southern Sotho
Sundanese
Tamil
Telugu
Temne
Tibetan
Tigirinya
Tsonga
Udmurt Ukrainian
Urdu
Venda
Votic
Western Frisian
Yoruba