looking back at this post, there seems to be a tiered system of "what languages have resources" which directly correlates to how economically important that language's speakers are to the global economy. i'll layout a rough estimate (also note, this is coming from an English language resources only perspective):
1. the classic Anglo-centric langs. French, Spanish, German, Latin. All with large speaking populations and important culturally to especially the US and UK. many diverse resources using many different pedagogy.
2. the powerhouses. Mandarin, Japanese, Korean, Russian, Italian, Portuguese. representing very economically important languages, and to some extent, heritage languages of immigrants in the US, as well as lots of cultural influence for some. many resources, but usually using the same pedagogy.
3. the minor powers. Dutch, Greek (modern and ancient), Polish, Tagalog (Filipino), Thai, Hebrew, Arabic (usually only MSA though), most non-russian slavic languages, most norse descendant languages. mostly important to heritage speakers OR "niche" economic interests. fewer resources, but diverse enough to find a good fit for one's learning style
4. minor national languages. Burmese, Indonesian, Malay, Khmer, Laotian, Bangla, Nepali, Hindi (at least in the US, Hindi resources are sparse as hell), Farsi, Turkish, Kiswahili, estonian, finnish, Latvian, Lithuanian, Ukranian, Cantonese. Most national languages of central, southeast, and west asia reside here. Limited economic influence and vast english-speaking immigrant communities mean very few resources for prospective learners. maybe 2-3 good books/websites at most.
5. the neglected kids. daughters of empires, these languages number many. they have plenty of documentation, but their countries abandoned them for foreign languages. Belarussian. Igbo, Yoruba, Lingala, !Xhosa, Zulu, non-Tagalog major languages of the Philippines, non-hindi languages of India. These languages have been suppressed, neglected, derogatized, and more. Many are either sub-official, or not official languages of their home countries. Few, if any resources.
6. The basement. These make up the majority of the world's languages. The nearly forgotten masses, many are endangered due to cultural assimilation, hostile governments, and more. Usually no resources at all besides the community that speaks it. Maybe. If you're lucky, you'll find a good article on the language on JSTOR or smth.
7. The dead. Those lost, but reviveable languages. Many indigenous languages of the US and Canada fall here. some books, but most are terrible sources. contact the communities instead.
8. The extinct. Those lost, and unrecoverable.
feel free to give input and refute any placements i made. this is a rough estimate by any account.