For similar reasons re: canonically-English titles, I wouldn't put a romanization on non-name description like "サバンナからの女の子". Japanese readers already know how to pronounce it, and, unlike with names, English readers have no reason to.
The Pixiv Encyclopedia page about the characters in that sequence also uses Tibet-chan -- that's established fanon, definitely go with it. It describes the other two as "アフリカ先住民の少女" ("indigenous African girl") and "イスラム風衣装の少女" ("girl in Muslim-style clothing", I think). If you want to identify them by "some way they've been described in Japanese fandom", a wiki page that's been edited by lots of users seems a more likely source than a tag that one random user has put on one picture.
Re: This should be all the Japanese ones
The Pixiv Encyclopedia page about the characters in that sequence also uses Tibet-chan -- that's established fanon, definitely go with it. It describes the other two as "アフリカ先住民の少女" ("indigenous African girl") and "イスラム風衣装の少女" ("girl in Muslim-style clothing", I think). If you want to identify them by "some way they've been described in Japanese fandom", a wiki page that's been edited by lots of users seems a more likely source than a tag that one random user has put on one picture.