Location: Italy, Venice maybe.
Language 1: Contemporary Italian, Venitian I guess?
Language 2: Contemporary Latin.
Skill: Master's degree in Chemistry, thanks.
Although reliant on a few technologies, I figure chemistry is a good choice. Italy at the time had a big glass making industry, so I'd be able to get some laboratory glassware made up, of varying quality. Coppersmiths would be useful for me too. One problem would be a supply of reagents, but as long as I could identify local natural sources, I should be able to synthesise the main stuff.
With that, I could do all kinds of shit. Make modern fertilisers, introduce distillation of alcohol and water, set up a soap factory, make explosives.
Distillation of alcohol would probably be my first aim though. That'll get me rich enough to pursue other things. Fertilisers will probably help too. Soap and distilled water would improve the living conditions of people, but it probably will only get me a good reputation.
Explosives, I'll probably keep under my belt for the time being. I could imagine people reacting with suspicion and calling me Satan or some shit, but the others are all good, health-giving things, so I think I'd be safe with them. But if I get into a tight spot, I'll rain down God's judgement on a motherfucker, I swear.
I could probably also help out with iron working with some of my knowledge. I don't know actual metalworking or engineering, but I would know eg. the relationship between carbon and steel, as well as adding other things like silica, etc. so I could at least push metalsmiths in the right direction.
All those ventures are already probably enough for one lifetime's work, so I'll leave it at that.