I do NOT want to exclude other languages. Bookcrossing is a global hobby that should never limit itself to nations or languages.
True...but I believe there are some languages that don't have a word for "the" - for instance, as far as I know, Japanese doesn't really.
Can't tell about asiatic languages.
Generally I would prefer that everything should count which is the _equivalent_ to english "the", which is defining a definite form. I don't say that this is the only possible internationalization of the challenge that makes sense, and RockDg9 IIRC never stated anything about that point. It _might_ make more sense to require a specific word (or set of words), but if you would do so, you would exclude many languages in which the definite form is not expressed by prepending an article, but by some other grammatical construct, and I wouldn't want to exclude anyone.
Generally awareness about differences in cultures and specifically languages is probably higher in Europe than in America. Everything is rather close here. If we feel like seeing something different than "good old Germany", France and Switzerland and Austria are all only about three hours by local train away, using an affordable group off-peak ticket. One day on the train or bus takes you to a choice of 20* independent countries, most of them with their own languages (* this includes four "dwarf countries" but not semi-autonomous territories like the Channel Islands).
Since I started bookcrossing, I have released books in 19 different countries, with 17 different official languages, and none of those places I have reached by plane. We went everywhere by train, bus and regular ferry (took a plane back home once, though).
I actually keep a stock of books in various languages to release in case we go "there", wherever "there" is. Since I plan to send off a box with mixed languages to a fellow bookcrosser next week, there probably will be various versions of "the" on the list ;)