Cambodians call foreigners as Barang, especially if they are gringos (North Europeans and North Americans.) But they assume that any Non-Asian person is a Barang. It corresponds in the Thai language to Farang. The origin is highly discuss but it is not derogative at all. It seems to come from Barangsaes (French or Français.) It is likely possible, because Khmer language did not have the phoneme F (now, thanks to the French influence, it has it.) It is also possible that Thai language was influenced by Khmer through the permanent cultural interaction between both countries.
Now well, according to my observations, the origin of Barang from the French word Français is the only possible origin. The prove would be that we say Proteh Barang (ប្រទេសបារាំង) for France; Piasah Barang (ភាសាបារាំង) for French language and Chunchiet Barang (ជនជាតិបារាំង) for Frenchman. There you are. In this case, before all Cambodians, any Western becomes a Barang meaning a Western. It is the only plausible possibility.
There are other hypothesis, but I do not find much connection. However, they must be contemplated if you are interested in getting the origin of the world Barang.
In the Malay language Barang means Product. The question is if this Malay Barang is the same Khmer Barang. How is it possible that a Product becomes a Foreigner or a Western, although it has any connection with capitalism. Anyway, I have always complain that several Cambodians – and Thais – look Westerns as Walking Wallets, in this case it would have a Malay meaning 
But I would say that the two words are just a coincidence.
The relation with the Thai word Farang is better. As Thailand became more popular in the West than Cambodia – at least until now -, I have heard many Thai resident’s Barangs concluding that the Khmer Barang comes from the Thai Farang. It happens also with Muay Thai. Studying the origin of Farang in Thailand, it is assumed that is a derivation of the Indo-Persian word Farangi that means Foreigner. It is a plausible concept. It is not difficult to trace its importation into the Southeast Asian Mon-Khmer languages, including Malay.
This COMMENT of “Adolf Hitler” was removed for obvious reasons and I clarify, the use of “Gringo” in this post was made without the intention to offend. Actually the expression is widely used by US citizens themselves as they are mostly called in Latin America. At the same time, it was used as a meaningful comparison to say that Cambodians (and Thai) use the word “Barang” or “Farang” to Western foreigners, as Latin Americans use “Gringo” and both ways are not derogative, at least the speaker intend to do so. For the word “Gringo” Wikipedia has a good article about, and the Urban Dictionary (though take care with some concepts there made by any editor) and Dictionary.com that says that a Gringo is (in Latin America or Spain) a foreigner, esp. one of U.S. or British descent.Therefore, it is the same value of Barang for Cambodia, that was the concept I want to express.
You are welcome to comment, even to oppose to my own views and you are going to have space in my blog, but in a respectful way.
Please read “The Old Gringo” of our dear Ernest Hemigway, here an essay of Carlos Fuentes.