The biggest issue I have when designing adventures is the same one I have when designing characters as a player: naming them. I know what I want their skills, history and personality to be, because that all has a tangible effect, but a name can be anything. If it can be anything, how do I narrow down what it SHOULD be?
It gets even harder when designing adventures with multiple characters, all needing a name. I can't share names and have fifteen Jims, but I don't want to end up with the three human tavern owners Jim, Nunzio and Kxarbutlko. Different names that sound like they fit together... Very tricky, personally.
And I don't wanna just use foreign words, because I live in fear of the day when a Spaniard asks me why I named a villain "cabbage" or something. And it's surprisingly hard finding a relevant word that actually sounds like a decent name.
I usually take a look at names from one language and choose those I like the most. Recently I held the game in another random fantasy world where PCs were new and I made sure my players don't have a lot to do with French and gave every NPC a French name. But they can be hard to pronounce and to note by players if you don't know it.
Good picks are Nordic names - Norwegian/Danish/Swedish - because of popcultural references. I'm thinking of making repository of semi-Polish fantasy names for all those who like Slavic themes but there is a long way before me.