Hey there. Basically I have a forum around the same size as yours. I moved from myBB to NodeBB.
I am using NodeBB since almost a year now.
Security isn't an issue. There have been some XSS vulnerabilities here and there, but these have been patched within a few hours.
Updating NodeBB is easy as well, you just run git pull, stop your forum, run ./nodebb upgrade and then start it again. No need to restart NodeJS or Redis/Mongo.
Setting up a system running PHP and NodeJS is fairly simple. Maybe take a look at my tutorial "High performance stack".
Moving NodeBB is pretty simple too. Just copy all the files to your new machine and run ./nodebb start. Thats it (assuming you have your stack installed of course).
NodeBB automatically chooses the right language for your user. At least my members got their mother language. Alternatively the users can set the language in their profile settings.
My forum is using a Redis database. It takes around 500-700MB RAM, not a big deal for me, but depends on your server of course.
To contribute to the NodeBB translations you can check Transifex and participate.
If you want I can help you doing the movement. Feel free to drop me a message.