Portugal migration

My two parents have wanted to move to Portugal for so long but have always postponed it.

What exactly are the immigration requirements before BREXIT? As far as I'm aware, they need to fly there, rent somewhere for 3 months, then file for permanent residency, is this correct or am I mistaken?

Can they just rent somewhere?

They're looking to move to the Algarve but are worried about the language barrier regarding work etc. since my father has a career of 30 years in the UK and is worried about leaving it for such instability and uncertainty.

Thanks!

While EU/EFTA citizens have the right to freely move to Portugal, they must request a registration certificate if they stay for longer than three months. EU citizens can also request a permanent residence certificate after five years.
The situation for UK citizens may change following the UK’s exit from the EU in the coming years; until then, however, UK citizens can enter and remain in Portugal without a visa.