Are there any spells (mantras) in Hinduism which are not in Sanskrit, but in other languages. I came across a mantra in Awadhi dialect of Hindi, it's something about blessings for studying and was devoted to Lord Rama! Do you know any other spell which is not in Sanskrit? Why can't we have spells in other languages? Any example?
In South India,in the temples of the Lord Vishnu, the lord is often worshipped using tamil verses which were written by the great saints of Sri Vaishnavism Azhwars.Most important and famous one is the Thiruppavai which was written by Sri Aandaal(maiden name"Godha devi").Thiruppavai is recited in almost all vishnu temples in the month of Dhanurmasa in the place of suprabhaatham.There are plenty of traditional works of azhwars that are chanted in Vaishnava temples during different occasions.
I know of few mantras which are in Tamil. They are siddha mantras(given by the siddhas).Few examples are as below:
Ganapaty-Om mahaganapatikku harohara. Parasakti-Om Parasktikku Harohara Shiva-Om Shiva Perumanukku harohara...and similar mantras for many other Gods.
Edit-I'm adding the source which contains the whole list of such Mantras.Also adding a few more of these Mantras for some important Deities.
MahaGanapati-Ohm Maha Ganapathiku Arohara
Shiva-Ohm Sivaperumanuku Arohara.
Vishnu-Ohm Vishnuperumanuku Arohara.
Parashakti-Ohm Parasakthiku Arohara.
Brahma-Ohm Birumma Devanuku Arohara.
Asta Lakshmi-Ohm Astalakshmikaluku Arohara.
Saraswati-Ohm Kalaivaniku Arohara.
Skanda(Murugan)-Ohm Murugapperumanuku Arohara.
Navagraha-Ohm Navakiraha Nayagarkaluku Arohara.
But,since i don't know tamil i don't know the exact meanings of the Mantras.It will be nice if someone who knows tamil can explain the exact meanings of the words that are being used in these Mantras.
It's hard to answer this question simply because the word "Mantra" in itself is too broad, in my opinion.
First of all, lets try to understand what is meaning of Mantra.
Mantra(मंत्र) is a sacred utterance believed by practitioners to have psychological and spiritual powers. The Sanskrit word mantra consists of the root man- "to think" (also in manas "mind") and the suffix -tra, designating tools or instruments, hence a literal translation would be "instrument of thought".
So, Mantra(मंत्र) = instrument of thought
Next, these Mantra comes in many different forms. For example: verses from the Rigveda or musical chants from the Sāmaveda can be considered as Mantra.
Beside this, there is no formal definition of Mantra.
Considering all above points, its hard to prove or disprove what is Mantra and what is NOT a Mantra.
Now, to answer the question, there is Maruti Stotra written in Marathi, which is popular in Maharashtra India.
Other popular example is Hanuman Chalisa written in Awadhi language.
These both can be considered as stotra (and not Mantra) but one can use either whole or some verse inside it, as an "instrument of thought".
And hence it can be termed as Mantra.
Certainly there are non sanskrit mantras as well. If you look at Natha sampradaya, they have tradition of Shabara mantras which contain hindi, sadhukkadi, avadhi, and urdu words in addition to sanskrit words.
After all mantras are based on the intention, concentration and belief of the chanter and based on the sound rather than any specific language.
There are many folk tales and tradition which talk of non sanskrit mantras. In kerela too there is a section called Kerela mantrikan which has non sanskrit mantras. In some tribal population we do find mantras or spells of different language.