Is there currently a country (or have there been such countries in the past) where the government is planning to introduce English as the official language, despite the fact that the local population is not Anglophone and has never been under Anglophone colonial rule?
Rwanda is such a country, having made the switch from French, the colonial language under Belgian rule, to English, in 2008.
The country has also joined the Commonwealth of Nations, despite never having been subject to the British crown.
I'm unaware of any country with current plans to follow in Rwanda's footsteps.
Tonga's official languages are Tongan and English. Tonga was under British influence for a long time, but it was never a British colony, having become a protectorate only, though it later joined the Commonwealth of Nations. The Tongan monarchy was never abolished.