"a cow is big."
Translation:Tha bò mòr.
Hmm, I have a different theory (I won't fact check, so I apologize for any mistakes). In old Latin a cow was "vacca" and obviously in Romance languages that word is still present - Portuguese, Spanish, Catalan spell it "vaca". In French, which is a bit "farther up", there is "vache", which still derives from the old Latin word, but is a bit different, perhaps sharper if you will. It is known that insular Celtic languages (Breton, Cornish, Welsh, in that order) were influenced by French - in Breton a cow is "buoc'h", clearly derived from French, and in its sister language Welsh it's "buwch", still very visible having connections with "vache". The Celtic languages influenced one another as well, and especially so during the Middle Ages, so I don't find it impossible that a word has again traveled a bit farther up to Scotland and shortened its form to "bò", absorbing the regional pronounciation, which was markedly different from Welsh. It is well to remember that all of these would have, despite those differences, been pronounced relatively similar. Welsh and Scottish Gaelic were still predominantly oral languages at that time, and a peasant was much more likely to use the word for cow than, say, a scholar or a poet. So my theory is that Celtic languages borrowed the word from Latin via French, and that European words for cow (or bull) derive from the same "co/ko/bo/vo" or "ca/ka/ba/va" root, which changes regionally according to spelling and pronounciation, thereby also connecting English "cow", Dutch "koe", German "Kuh", and so on. As for Vietnamese, as it has long been a French colony, I wouldn't find it unlikely that they borrowed their word for cow. After all, their words for apple ("bom"), chocolate ("sô-cô-la"), salad ("xa lát") and cake ("ga tô") are all French. While most world languages use some of the variants of "moo" to describe a cow's sound, there are also those who don't: in some South Asian languages such as Bengali - which still have some Indo-European roots - it can be said as "hamba" or "ungaa". Also the Vietnamese expression for mooing ties all of these - French, Celtic, etc. - together in a way: "ụm bò". So, my point is I don't believe this was really an onomatopoeic word. Hope this wasn't too far-fetched! :)
There are several points here.
Firstly it is better to do at least a little research rather than posting and leaving it all to others. Looking up vacca, cow and bò in Wiktionary, then following through the etymologies and the descendants to find all the cousins would be a good start. If you do that you find that vacca is said to come from PIE *woḱéh₂, but if you follow the link to that word then it says that vacca is a descendant if not loaned from a Celtic language. So we immediately find the origin is disputed. Cow and the Celtic words are said to come from PIE *gʷṓws ’cattle’. This seems to be the more productive PIE word, leading to a load of words in different languages. The gʷ is the interesting part as that suggests they think it was a sound that could give rise to either a labial or a velar, as shown by the descendants.
When I said I thought it was onomatopoeic I did not mean that each word was. It seems highly likely that any language, way back, that did invent a new word would use some representation of the sound, but then this would be modified as words do evolve over time according to the rules of various languages. I find the fact that essentially all words suggested in Wiktionary have a voiced labial somewhere highly significant though.
As for the descendants of vacca, the cc was a sound distinct from c but it ceased to be, so that would explain the change to vaca. The French ch is a specific sound change that occurred in what became standard French that distinguishes it from Norman French and other languages. This means that we can tell if a word comes from standard French quite easily – so chandelier comes from standard French, but the related candle comes from Norman French. So vache is a quite logical descendant of vacca. But Breton is different. It is a Celtic language with a lot of borrowings from French. This led to two different chs. One in Celtic words, pronounced the same as in Gaelic, and one in French words, pronounced the same as in French. This led to confusion as to how to pronounce it. So some priest in the middle ages decided to clarify matters and to write ch for the French one but c’h for the Celtic one. So when you see buoc'h you know that it is pronounced the Celtic way. Since there is no such sound in French you can be confident this is not a French word. Given this and the Welsh use of w as a long o, the Breton buoc’h and and Welsh buwch are essentially identical. Given also that these British migrants were farmers they would have been using the word from the minute they arrived – they must have had buchod with them on the boat if they were planning to farm – long before they came into contact with any French speakers.
In general, all the words they used from when they arrived would have been Brythonic, with French used for any new words they needed later. Your examples of ‘chocolate’, ‘salad’ and ‘gateau’ illustrate this as they are all examples of modern cuisine and are substantially international words. Your example of bom from French pomme concerned me as pomme does not come from the most common Latin for ‘apple’ which is malus. It comes from another word, of obscure origin. That means we cannot rule out the Breton being a cousin not a descendent of the French. It seems to have existed alongside other words in Welsh and Middle English.
As for the Vietnamese, there are two reasons why it seems to me to be unlikely that the word comes from French. One is the fact that the Vietnamese were farming cows for several thousand years before the French arrived, and it is rare for colonizers to provide names for things so basic to life. This is precisely the reason given for English not having adopted the French word when England was colonized by the French about six centuries before Vietnam.
The second reason is that we can easily find the origin of the Vietnamese word, traced back to Proto-Vietic which is at least a couple of millennia old, which has descendents in many languages that were clearly not influenced by French.
Of course cow-farming was not invented in either Vietnam or Scotland, so it it imported technology. It is possible it came from the same place, many thousands of year ago, and that the word came from there too.