I am analysing the global meanings of straightness for a hair care
company, from a culrual perspective.

I am interested in receviving examples of what straightness means in
Thailand, China, Mexico and France. Are their storng commonalities or
differences here? What are the popular sayings around 'straightness'?
What slang assocaitions might they have in each territory? What are
the most striking popular cultural examples and images of straightness
in each region?

I would suggest you split it into four questions, one for each
country/culture you're interested in, because it's very unlikely that
the same person be aware of peculiarities of such different and
distant cultures, while it's quite probable that among the researchers
there are individuals from some of those places (not me, anyway).