It was just the language barrier (probably still present a bit - not sure how well Dembélé communicates in Spanish nowadays but I presume it is going in the right direction)
I agree.
However let's put something straight regarding the "language barrier" - that you have also mentioned:
The international and common language of football is English. At least it is in Europe and in regard of the the leading teams, which nowadays most often comprise many different nationals.
Just as an example: My own country's national team has the 4th manager in a row, who is not from the country and all four of them had and have been speaking to the players in English.
Let's just bring PSG's example here. The coach is German and speaks no French whatsoever.
The assistant coach for instance is a fellow countryman of mine (Zsolt Lőw) who is also a personal acquaintance of mine, a very nice guy. No, we aren't fiends as such, but crossed each other's way several times in the recent two decades.
Zsolt for instance worked in Germany before PSG, speaks excellent German and English, but like his boss: no French as yet.
Quite recently he gave an interview to a local paper and was asked how he speaks to Neymar, Alves or Cavani? Of course in English - he answered - and that's the most common and normal thing in Europe these days.
I have no idea how much English Neymar knows, for instance, but I'm sure the French papers are not filled with shit questioning Neymar's French capabilities, although I'm sure he didn't necessarily learn more than a couple of words or perhaps some sentences in more than a year.
Thus if a young and inexperienced player like Dembele doesn't know the local tongue, or slow to imbibe it, that's very normal. Barca is one of the leading clubs on the globe and hence it should be as international - even in this regard - as possible.
Neither average humans, nor footballers have equal language skills and talents, thus for Dembele it is as much of a problem if he doesn't speak English, as his lack of Spanish... but there are French people in the team to help him out, if needed.
At least he doen't have a pro translator like Arda Turan used to have...