In British English, the spelling “program” is widely used nowadays for IT but “programme” is still the norm for other contexts.
Simple enough. But it does occasionally mean that a single document can in fact legitimately use both spellings, and customers and agencies are quick to point that out as an error. You’ve got to treat them as separate words, almost.
- a television programme
- the programme of events for today is…
- the program suddenly gave me a Blue Screen of Death
- The same applies to the verb form, on the few occasions the conjugations wouldn’t be identical:
she programs a computer
I programme the conference
American English is easier: that only ever uses the spelling program.
Prevalence: low. Non-native authors don’t like mixing the two, understandably.
Frequency: low. Automated QA programs and spelling checkers can get confused.
Native: no. Though older UK dictionaries might disagree (and insist on the short form being purely American).