When multi-classing, do you gain any other proficiencies outside of weapon and armor. In particular do you gain saving throw proficiencies?
The list of proficiencies you gain when multiclassing are listed on page 164 of the players handbook. They do have several examples of classes that grant non combat based proficiencies. Rogues, Rangers, and bards all give a skill or tool proficiency.
Saving throw proficiencies, however, are not mentioned on any of the character class entries, and due to how specific the list is, we can assume that this is intentional.
So yes, other proficiencies are available, but they are class specific. No, no class grants proficiency in a saving throw for multi-classing.
The only apparent ways to gain new saving throw proficiencies is through a feat or through the 14th level monk ability Diamond Soul. The rarity of the ability to gain new feats coupled with the fact that saving throw proficiencies are expensive (one proficiency per feat) shows the importance and value of Saving Throws to 5th edition.