Hi John,
I spent the summer there on a business trip perhaps similar to yours.
Few in the general population speak English or other European languages. As I do not speak Arabic, to get around town on my own I asked someone to write the destination on a piece of paper that I showed to taxi drivers. Worked fine.
Tripoli is a friendly place, smiles and interest with foreigners etc. It has been a closed off place for a long time, so people may be curious, in a friendly way.
Only a small part of the city in and around of the old town is good for walking on foot. I ventured into the old town several times. It is a maze, full of small bazaars and shops. You will find lots of young men from south of Sahara there, and they are much more likely to speak English. These people tend to get the very worst jobs, and often try to continue to Europe. It was fun to chat a little with them, and I was welcome to take photos afterwards.
I do not recommend you to take candid street shots, especially not of women, which is regarded as offensive. People will get angry with you. But it often is ok to photograph the men after a chat or after announcing your intention without words. Architechture and nature is of course ok to photograph.
Do visit Leptis Magna, which is a beautiful roman ruin city just a couple of hours drive from Tripoli.
Try to get in touch with the small diplomatic community or the community of foreign consultants. I found them very welcoming - they are few and I guess that they enjoy find someone new to speak to from time to time.
What other countries do you usually work in?
Please let me know if there is anything else I can help you with.