At 11 weeks pregnant, a 13-hour flight is generally considered okay by most doctors, but honestly it's quite long and the risk of blood clots goes up with prolonged sitting, so it really depends on your individual pregnancy and health. I'd say check with your gynae first because they know your situation best and can give you the green light or suggest precautions like compression socks or getting up to walk around every couple of hours. Most airlines also have their own policies on how far along you can fly, so worth checking with Lufthansa or whoever you're flying with too.
Yes generally safe. But nausea and exhaustion will make flying uncomfy. There is a higher chance of dvt risk since this is a long haul. (Anything more than 4 hours have higher risk): so make sure you get compression socks, walk the aisle every hour, drink lots of water as cabin air is very dry and wear your seatbelt low across the upper thigh and not your abdomen. Do also ankle exercises while seated. Flex then, rotate them etc. Enjoy germany!!