Indonesia does recognise limited dual citizenship. It’s easy to see that fact online and assume it applies more broadly than it does.
Law No. 12 of 2006 on Indonesian Citizenship grants limited dual citizenship specifically to children of mixed Indonesian/foreign marriages, and only until they turn 18 (or 21 in some readings) — at which point they must choose one nationality. This provision exists to give children a grace period, not to let a working adult hold two passports indefinitely.
For an adult Indonesian citizen who voluntarily acquires a foreign citizenship — say, through naturalisation in Germany — the general rule under Indonesian law is loss of Indonesian citizenship. The mixed-marriage-child provision doesn’t extend to this case. If you’re weighing naturalisation abroad as an adult, plan around losing your Indonesian citizenship, not keeping it.