Yes, it's a difficult one. I mean, a car is effective for getting from Gothenburg to Stockholm but it may not be the most efficient way to travel (petrol, time, cost etc). The meanings are quite different.
Try translating this Peter Drucker quote into Swedish: "Efficiency is doing things right; effectiveness is doing the right things." :)
Actually, Swedish often combines words to convey the meaning. Regarding the word "effektivitet", it can be combined with other words so the ambiguity is removed in the context eg tidseffektivitet and resurseffektivitet Obviously, these two are translated as efficiency because they have their counterparts in English.
In recent years, there have been a lot of international conferences, university courses, business plans" that treat Effectiveness and Efficiency as two different areas (sometimes overlapping). This has resulted in Swedish academic literature, universities (eg Chalmers) and businesses opting for separate terms for effectiveness and efficiency:
inre effektivitet = efficiency
yttre effektivitet = **effectiveness"
So back to the Peter Drucker quote at the start. The Swedish equivalent might be: "Inre effektivitet är att göra saker rätt; yttre effektivitet är att göra rätt saker."
Finally, a website that explains the two English terms rather well: https://www.publichealthnotes.com/differences-effectiveness-efficiency/