sole - won't work, because taban is only a sole of a foot, not of a shoe
arch - nope, arch is anything that's shaped as a section of a cicrle or similar to it - in architecture or elsewhere. Taban is the sole of a foot.
"tek ovog puta" can't be translated as "only this time" (as every online translator will say), because it doesn't mean that at all. It means "never before until this time" and does not imply "this time only"
top of head - one's or none's shouldn't make a difference
crown of head, but should work for republics as well
center of mass
Actually "a woman who is pregnant" is correct. The trouble is that you won't like it, too long. If you ever come across a word which means exactly that as its primary meaning, let me know.
alien - doesn't work, it covers only a part of the meaning; "tuđ" is not just alien, it is "belonging to an alien" or to anybody else but the subject
belonging to someone else - is as close as it gets to "tuđ", and just as elegant.