I'm a barang. So are you.
It's a Khmer word which means foreigner, especially someone of European origin. It is not considered an insult. Usually... The origin of the word is a bit convoluted. In Thailand the word for foreigner is farang. The two words are obviously related. Farang came first. The Khmer word is different for the simple reason there is no F sound in the language. I know this first hand. Often people here struggle with my name. There are people here who know me as Jepp or Jess.
On a similar note, in Asia the R and L sounds are often confused or interchangeable. The word should probably be transliterated as balang. I have seen the Thai word spelled falang.
The explanation for the word farang is that it is a corruption of the word français. The word became part of the language through interaction with French traders. This is the most popular explanation and seems to make sense. It is also almost certainly incorrect.
Not only did other European traders (Dutch and Portuguese) arrive before the French, other non-Europeans did as well. Persians arrived before Europeans. The Persian word for foreigner is farangi. There are other explanations but this seems the most obvious.
Why, then, the common belief that the word comes from the French? To me it seems to appeal to a typically white/European-centric view of the world.
Persian is the language of Iran. Most westerners think of Iran as an Arab country but it is most definitely not. Only three percent of Iranians are Arabs. Even less speak Arabic. It is a huge pet peeve of Iranians that travelers who believe themselves to be worldly and educated show up in Tehran and try speaking to the locals in Arabic. Imagine someone getting off a plane in New York or San Francisco assuming the people there speak Arabic and you get the idea.
The majority of Iranians are ethnically Persian and speak Persian. Do white tourists from Europe, America, Australia and New Zealand want to be referred to a word from Iran, of all places? It's bad enough being called French!