If you look up 'squatters' in the thesaurus, the only other reasonable equivalent is 'homesteader', but that's a very specific case, and presumably you do not have the information to know if they qualify as such - nor does it really matter.

Rather than try to rummage for specific translations, I'd suggest that you go for the general idea.

In my experience, 'proprieters' refers to businesses, and in this case we're talking about landowners.

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Note added at 7 hrs (2010-02-06 05:27:36 GMT)
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In Marina Orellana's "Glosario Internacional," 'squatter' is "ocupante."
The difference between your terms is likely to turn around their relationship to the owners of the land. The distinguishing characteristics are the arrangements under which they occupy the land.

14,400 refs for "non-landowners":

grazing rights to non-landowners was distinctly more restricted, ..... whether or not 'squatters' are closely related to the landowners (see Cheater, 1982). ...
www.jstor.org/stable/1159711

All the Pakhtuns of an area may combine to exploit the non-landowners by demanding ... The non-landowners are already living barely above subsistence level, ...
books.google.com/books?isbn=0631224408...

black population resided as tenants or ''squatters'' on mostly African ..... land for both landowners and non-landowners; the decrease of stock rates and ...
www.informaworld.com/index/910528365.pdf -

... the more non-landowners lost pasture and common rights and forest access. ... Gypsies, amongst those squatting on the forest's margins, are also linked ...
issuu.com/didimenendez/docs/shwa18miporadio