I’ve been researching this topic a bit as there is an extensive number of languages and dialects, and even working in the industry for almost 4 years, I still come across new fascinating languages on a regular basis. I don’t think it’s possible to assess with great accuracy which language is the most expensive to translate, but let’s review a few facts.
There are many languages (especially African languages) which are still underdeveloped and you will be surprised to hear that a language so widely and frequently spoken as isiZulu is still a developing language. We are lucky though, as resources are quite common and readily available for this language and, we are blessed to have so many reliable and trusted African resources that we can offer these underdeveloped languages at affordable rates but still guarantee high quality work.
On the other hand, you have languages from developed countries for which linguistic resources are very hard to come by. One good example would be Norwegian, especially translation work into Norwegian poses its own challenges. The reason why rates for translation into Norwegian are exceptionally high is because Norway is an expensive country. It is now established as a well-known fact in the translation industry that Norwegian is the most expensive language to translate. Perhaps a way to fix this issue is to have a few linguists living in another country?
Anyway, from isiZulu to Norwegian, we can easily say that being a translator is no easy task and they deserve every penny they charge!