The answer lies on page 163-4 of The Player's Handbook:
Your proficiency bonus is always based on your total character level, as shown in the Character Advancement table in chapter 1, not your level in a particular class. For example, if you are a fighter 3/rogue 2, you have the proficiency bonus of a 5th-level character, which is +3.  - The Player's Handbook (2014), Page 163-4.
There are multiple examples of classes that possess non combat based proficiencies (i.e. outside of armour and weapon proficiencies). For example, a Bard would have a skill or tool proficiency. Though this does not include saving throws.