Why do we borrow words from other languages?

All languages contain loanwords. Some languages, such as English, have more of them, other languages, such as German or Icelandic, have few, and other again, such as Swedish, have a moderate level of loans. Why is this the case? The answer is very complex and is a combination of past and present history, geography, language size and power, and language structure. As a rule, languages are affected by contact from their neighbours. No language – and no part of language – is totally “loan-proof”. Any word in a language can potentially be replaced by a word from another language. But why? Why would a language replace a word for ‘mother’, ‘finger’ or ‘sky’, when they already have a word for this object (which all languages actually have)?
There are large differences between languages. Languages with more grammar, such as German or Icelandic, are more reluctant to borrowing – the grammar system of these languages run a risk of breaking down if the influx of loans is too large. Languages with lesser grammar are more open towards borrowing.
There are large differences between words. Words for modern cultural phenomena, such as computer, tea, or latte, are loanwords in almost all languages. There are languages that are exceptions, and these are typically minor languages, which naturally do not like to be overwhelmed by foreign words: these languages run a risk of disappearing anyhow. An example is the word for ‘radio’, in which Swedish borrowed the English term radio, whereas Icelandic interpreted the word as útvarp ‘out-throw’. The word for ‘airplane’ in  Scandoromani comes out as sasster-tjirklo ‘iron-bird’, and ‘webpage’ as khereske-rigg ‘house-of-side’.
On the other side, we have words that are almost never borrowed. Here, we find kinship terms, body parts, numerals (mostly), words for basic bodily functions and sense perception terms, words for natural phenomena (‘sky’, ‘stone’, ‘ground’). Linguists are particularly fond of these words, so-called basic vocabulary: these are good for almost everything in language, from investigating human cognition to establishing language families.
When it comes to cultural words apart from the modern ones, the issue is more tricky. Words that are inherent from a Eurasian perspective, meaning that they are part of the vocabulary for the cultural system of farming and pastoralism that has been present in Eurasia since the Neolithic or the Chalcolithic, are typically borrowed in languages outside of Eurasia (due to colonization).
We wanted to look at these words and compare them inside and outside of Eurasia. We looked at 100 words from the vocabulary of farming, pastoralism, hunting, and technology, which we compiled in 160 languages of different periods (4,000 BP-now) in Eurasia (Europe, Caucasus, Central and South Asia). We selected words that represented items that had been in use at least since the Chalcolithic, such as ‘wheel’, ‘axe’, ‘cow’, ‘horse’. It turned out that words were borrowed to a degree that was relatively high in Eurasia (12%), but that this level was highly diverging between languages. Compared to languages outside of Eurasia (as concluded in the WOLD project), the percentage of loans was low.  
The most interesting result was a significant correlation between language size and tendency to lend or borrow words in our corpus. Large languages were donor languages to all other languages, including other large languages. The level of mutual borrowing between languages decreased all the way down to the smallest languages, who were very frequent as recipients of loans, but which were almost never sources of loans, not even to other small languages. This demonstrates one of the most crucial causes for borrowing: power, defined not only by population size, but also by economic, material, cultural, or political power. Occasional exceptions of reverse borrowing of specialized words – think of lingonberry, fartlek, smorgasbord or ombudsman (Swedish loans in English) remain infrequent exceptions in the larger perspective.  

Presentation by Gerd Carling & Sandra Cronhamn at SLE 2018, Tallin.

Heat map demonstrating frequency of co-occurrence as Target (x) and Source (y) language defined as language size (1-5, with 5=larges and 1=smallest), based on 1308 loan events in 160 Eurasian languages spanning over 4,000 years of documentation (graph by Johan Frid).