I had the experience of this on one major occasion when working on the oil pumping plant near Amlwch in the 80s.
I was supporting my Manager (Swedish) as I often did then and he had taught me some swedish phrases to use for some things to use when he did not what the builders to understand if we had a problem or they were being awkward on purpose. We went into town to buy some food at lunchtime and check out a place for the team to stay. As we walked into the pub all conversation instantly switched from English to Welsh. Thorsten, who had encountered this before said in Swedish (I paraphrase) "these nice people are being less than polite and helpful", a statement he often made about builders, and I responded as I should and added shall we leave. As we turned to go everything switched back to English and the Landlord asked, in English, if he could help. Thorsten replied in his almost accentless English " No thanks, we know we are not welcome" as we left. It cost the landlord a good bit as he lost any chance of providing accommodation for our team 10 for 2 weeks whilst we commissioned parts of the Installation.