Because he was drunk and causing trouble again with the trollop of a barmaid. Also when Jamie interrupts and says “Honorable Wife” because he knew if Mr. W translated it he would have said Honorable First Wife. A dead give away to Claire that Jamie hadn’t told her something very important.
The book version of Mr W is much different than the TV Mr. W. In the books he is not nearly as fluent in English, an example of this is, in the book Mr.W’s pronunciation of Jamie is “Tsei-mi”. The book Mr.W has a huge foot fetish which has to do with the traditional binding of women’s feet in China. He is portrayed in the book as a more humorous and a less politically correct than one might expect these days. If they portrayed TV Mr. W the same as the book there would be crowds of people most likely screaming about stereotyping Asians. In the book his character fits the time period and is treated according to what might have really happened in the 18th century. No one in Edinburgh would have ever seen an Asian person before and they would have treated one with disrespect, distain and suspicion.