I do consider bilingualism possible if the Germans had founded their own counties, as they were entitled to - it could be similar to Switzerland.
The settlement probably mainly failed because the settlers spent most of their time looking in the interior for gold, getting diseases and making enemies in the local population.
A few more IQ-points for the bankers might have been sufficient to base the colony on all kinds of income instead of just going for gold - if the bank is just happy with getting a share of the local taxes and of the local companies founded on behalf of the bank.
A few people would still search for gold and die in the jungle. Others would be much more active in establishing farms, a harbor, a small city, and so on. That could easily lead to lots of Germans following, trade instead of fighting with most of the locals, and so on.
I don't know what the conflict with the Spanish Crown which finished the settlement was based on - but I suppose a little bit more success would be seen positive by the Spanish. At that time, they didn't need to worry too much about independence movements, and it would be profitable for them to make the colony attractive for immigrants, by allowing some autonomy in terms of language and the likes.