Hindi-medium study material availability:

Books (comprehensive):

• M. Laxmikanth's Indian Polity — available in Hindi ('Bhartiya Rajvyavastha', McGraw Hill)
• Ramesh Singh's Indian Economy — available in Hindi
• NCERT textbooks: All available in Hindi via ncert.nic.in (free PDF)
• Spectrum Modern History — available in Hindi
• Shankar IAS Environment — partial Hindi notes available; full book in English only

Current Affairs in Hindi:

• Drishti IAS magazine (monthly, ~₹120–299 print / free PDF)
• Vision IAS Hindi PT365 and mains materials
• ForumIAS EPIC in Hindi (selected topics)
• All India Radio Hindi news (especially Spotlight/Guest interviews)

Test series in Hindi:

• Vision IAS: Hindi-medium Mains and Prelims test series
• Drishti IAS: Full Hindi test series (Prelims + Mains)
• PW OnlyIAS: Hindi-medium materials expanding

Regional language material:

For Kannada, Tamil, Telugu, Malayalam, Bengali, Marathi, Gujarati — almost no commercially produced UPSC-specific study material exists.

Regional language aspirants typically:

• Study from Hindi/English sources and take notes in their regional language
• Use state PSC materials (which exist in most regional languages) as a base
• Form small groups for peer-created notes in regional languages
• Use translated NCERT textbooks from state education boards

Online resources: YouTube channels in Tamil, Telugu, Kannada, Malayalam covering UPSC GS exist (search 'UPSC [language]') — quality varies widely.