Just say it, don't obscure it.
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Why?

1. Your reason has value to others
2. Some general judgements are, in fact, justified
3. You are free to have an opinion, and are not forcing it on others (see [gnasher729's answer](https://workplace.stackexchange.com/a/54031/41917), too)

Your client isn't the foreign company in question.<br>
It is good if they know your requirements clearly - helps them match offers to you, not to mention it may help them avoid the same pitfalls.<br>
If you think it feasible, leave it to them to turn the offer down (or offer to someone else, or have the terms amended) with the needed diplomacy.<br>
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Cultures / subcultures would not be identifiable entities if they had no distinctive traits. Superstitions, ingrained habits, different moralities taught as a way of life...<br>
I would not try selling whistle-activated devices to theatre employees. I would not count on a Japanese businessperson clearly stating refusal. I would be extra wary of anything where a newly met Rom and my cash are mentioned in the same sentence.<br>
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Past experiences are a quite valid reason for making personal decisions - in many cases, they're the only form of statistics you have available.<br>
You have detected a trend. You are not obligated to have peer-reviewed social studies to put value on it. You do not claim that is representative of the country's population, only that you consider it sufficient to reject the offer in favor of ones you are less wary of.<br>