Do you lose a streak, if it's because of server problems?
Title says it all.
I completed a Czech lesson and didn't get credit, then did a french and didn't get credit. Now I did it on my phone on the app and all 45 XP appeared. So as stated below make sure to have a "Freeze Streak" Equipped because you never know when the web might malfunction and you lose a streak.
Seems to be working now, but hasn't updated my streak so keep an eye on it.
But yes, you will lose a streak, even if it is the server's fault. Happened to me and I lost 500 days before