Many Spanish players don't, that is the point.
Well, that's where the translator and teammates will help?
Also, the manager can still bark basic ass tactical instructions and Football terminologies in Spanish. Takes an hour to learn those.
Trust me, you guys are overestimating the language barrier. The only place where it actually hurts is media interaction and local fanbase connection, both of which are major distractions at Barcelona and best avoided by a foreigner. Can be handled better by a media/PR trained translator instead of an emotional naive Football guy(Football people live in their own lil bubble).
Bielsa speaks no English and could still comfortably coach Leeds and make them a total Bielsa team structurally.
Also, if Flick gets the job, he'll know it 2 months before his first training session. That is more than enough to learn basic Spanish. Look at RL9. He's pretty decent at communicating in it already.
You guys are really overestimating this problem.