This article is taken from Ari
For Indonesians like me, "Kalimantan" is simply the name we use to call the entire island known in international world as "Borneo", just like how we call your "banana" as "pisang". But apparently, some people actually thought that "Kalimantan" as merely the name of the Indonesian part of the island. It's understandable that foreigners to have this way of thinking, but what disturbing is the fact that even few Indonesians thought the same way. So I think it's important for us to discuss about the origins of these names.
Borneo is a corruption of the word "Brunai" as the British tried to pronounce it. In time, they use that term to call the entire island even though the Sultanate of Brunai Darussalam covered only the northern part of the island. Colonial Dutch occupying part of the island in turn caught on calling the island as Borneo too, a name they took from their fellow colonialist, the British.
While Kalimantan is the name that is closer to us, the natives of Kalimantan. According to the history, while the Portuguese called the island as "Borneo", the natives called the island as "Pulo Klemantan". According to Crowfurd in Descriptive Dictionary of the Indian Island (1856), "klemantan" is the name of a sort of mango, he added that the name had a quality of a folk tale. While according to C. Hose and Mac Dougall "klemantan" came from the names of the local ethnic groups; Iban, Kayan, Kenyah, Klemantan, Munut and Punan. In Natural Man, a record from Borneo (1926), C. Hose explained that "klemantan" is a new name used by the Malay. W. H. Treacher had a different opinion, he thought that the name came from "Lamantah" which means raw sago. Lastly, Prof. Dr. Slamet Muljana in his book Sriwijaya had a theory that "Kalimantan" was derived from Sanskrit "kalamanthana" which means "an island so hot it burns".
A 1857 record says that in his correspondence with the Dutch Resident, Prince Tamjidillah of Banjar Kingdom referred to the island as "Kalimantan island" and not "Borneo island". This shows that even though the name Borneo was preferred by the Dutch Colonial government, the term Kalimantan was more popular among the natives.
We could not do anything about how foreigners define "Kalimantan", the important thing is that we as Indonesians should realize that the term has always been used by the natives of Kalimantan and has a long history. Thus, we have the obligation to use it correctly. Now I don't understand why Malaysians stick to the name given byt heir colonialist, nor do I want to force English speaker to change the way they call the island, and I couldn't even do it if I wanted to.
We can't force them to call "Cologne" as "Koln", or "Greenland" as "Kallaalit Nunnaat", so I can't expect them to change for us too. Let's just call the island according to our own ways, but with it'd be nice if they acknowledge that "Kalimantan" is Indonesians' way of calling the whole island.
On the side note, though. There are few languages that refer to the island as "Kalimantan" rather than "Borneo", and I really appreciate them! Some examples: Eesti, Иронау, and Русский.