What is the link language among common people of South India?

Short Answer - there is NO Such link language.

I am from Kerala and has traveled/lived(job,studies) in different locations of South India.

• English could be the most preferred option. In city limits it will be enough.
• In Kerala and Tamilnadu most people can understand TAMIL.
• Similarly in Telugana, AndraPradesh and Karnataka people can follow Telugu.
• People might understand Hindi as well. Hindi is part of school curriculum, migrant labors, Television and Bollywood movies.

Which means if you know Telugu and Tamil you can communicate with most South Indians.

But if a Malayali need to communicate with a Telugu guy English is the easiest option.

South India is the area encompassing the Indian states of Andhra Pradesh, Karnataka, Kerala, Tamil Nadu and Telangana as well as the union territories of Andaman and Nicobar, Lakshadweep and Puducherry, occupying 19.31% of India's area (635,780 km or 245,480 sq mi).

The majority of the people in South India speak one of the four major Dravidian Languages Tamil, Telugu, Kannada and Malayalam. But there are many minor languages spoken, most of which are used in the tribal communities.

1. Malayalam - Kerala/Lakshadweep
2. Tamil - Tamilnadu
3. Kannada - Karnataka
4. Telugu - Telungana & Andra Pradesh
5. Tulu- Tulu is a language spoken by around 2 million native speakers mainly in the south west part of the Indian state of Karnataka and in the Kasaragod district of Kerala.
6. Kodava /Coorgi . The Kodava or Coorg languageis a Dravidian language and the original language of the Kodagu district in southern Karnataka, India. with 2 lakh people.
7. Badaga is also spoken Badagais a southern Dravidian language (Old Kannada language branch) spoken by approximately 400,000 people in the Nilgiri Hills of Tamil Nadu. It is known for its retroflex vowels.
8. Irula is a Dravidian language spoken by the Irulas who inhabit the area of the Nilgiri mountains, in the states of Tamil Nadu and Karnataka, with 2 lakh people
9. Kota is a language of the Dravidian language family with about 900 native speakers in the Nilgiri hills of Tamil Nadu state, India. It is spoken mainly by the tribal Kota people. In 1990, the population was only 930, out of an ethnic population of perhaps 1,400, despite the great increase in the population of the area.
10. Toda is a Dravidian language noted for its many fricatives and trills. It is spoken by the Toda people, a population of about one thousand who live in the Nilgiri Hills of southern India with 1,600 native speakers.
11. Chenchu language is a Dravidian language which belongs to the Telugu branch of its South-Central family. This language is spoken mostly in Andhra Pradesh and Telangana states in India. This language is spoken by about 28,754 people (1981 census) of the Chenchu Aboriginal forests hunter-gatherer tribe. It is also called as Chenchukulam, Chenchwar, Chenswar or Choncharu.
12. Kurumba, also known as Kannada Kurumba, is a Southern Dravidian language of the Tamil–Kannada subgroup spoken by the Kuruba tribe. It is often considered a dialect of Kannada; however, Ethnologue classifies it as a separate language.Native speakers -180000
13. Koragas a Dravidian language spoken by the Koraga people, a Scheduled tribe people of Dakshina Kannada, Karnataka, and Kerala in South West India. The dialect spoken by the Koraga tribe in Kerala, Mudu Koraga, is divergent enough to not be intelligible with Korra Koraga. Native speakers - 14,000
14. Mukha-Dora (Nuka-Dora) is one of the Dravidian languages spoken in India. It is spoken by a scheduled tribe, who use Telugu as their primary language. It is spoken by the eponymous Scheduled Tribe in the state of Andhra Pradesh, India. Native speakers - 30,000
15. Koya is a South-Central Dravidian language of the Gondi–Kui group. It is sometimes described as a dialect of Gondi (spoken in Adilabad district in Telangana and in Gondwana region of Central India), but it is possibly mutually unintelligible with Gondi dialects. Native speakers - 360,000
16. Chenchu language is a Dravidian language which belongs to the Telugu branch of its South-Central family. This language is spoken mostly in Andhra Pradesh and Telangana states in India. This language is spoken by about 28,754 people (1981 census) of the Chenchu Aboriginal forests hunter-gatherer tribe. It is also called as Chenchukulam, Chenchwar, Chenswar or Choncharu.
17. Kolami (Northwestern Kolami) is a tribal Central Dravidian language used in Andhra Pradesh and Maharashtra states of India. It takes route from the central branch of Dravidian Language tree and falls under Kolami–Naiki group of languages. It is the most widely spoken Central Dravidian language. Native speakers -120,000
18. Maldivian the first language of nearly 10,000 people on the island of Minicoy in the Union territory of Lakshadweep, India, where the Mahl dialect of the Maldivian language is spoken.
19. Aranadan is a Dravidian language spoken by several hundred people, predominantly in the Malappuram District of Kerala state, India. It is similar to Malayalam and Tamil, and also contains elements of the Kannada language. It is also known as Aranatan and Eranadans. Native speakers -20,0
20. Kanikkaran is a language spoken by about 19,000 Kanikkars in southern India.They dwell in forests or near to forests in Thiruvananthapuram and Kollam in Kerala, and Tirunelveli in Tamil Nadu.
21. The Betta Kurumba language (Beṭṭa Kurumba) is a Dravidian language related to Tamil, Kannada and Kodava,and is spoken by 32,000 people in the Nilgiri mountains and in adjoining areas in Tamil Nadu, Karnataka and Kerala. Beṭṭa (ಬೆಟ್ಟ) means “hills” in Kannada

The largest linguistic groups in South India encompass the Telugus, Tamils, Kannadigas, Malayalis, Tuluvas, Kodavas and Konkanis, which, quite manifestly mirrors the diversification and overlapping of language cultures and customs.

According to the 2001 Census, Telugu possessed the third largest base of native speakers in India (74 million), after Hindi and Bengali. Telugu as such, was awarded the status of classical language in 2008.

Tamil was granted the status of classical language by the Government of India in 2002 and had approximately 60 million native speakers. Kannada possessed 38 million, whereas Malayalam had 33 million native speakers respectively.

There is no common language that the south Indians as a whole can understand.

Dravidian languages

http://www.ling.upenn.edu/~jason...

EDIT

Most of these languages are gradually dying.

List of languages by number of native speakers in India