I think if your first language is Indonesian then you will find that knowing English helps you a lot with learning German and Dutch. But learning German will help you even more with learning Dutch. (Or vice-versa.) Personally I would learn German first, unless you specifically plan to live in a Dutch- or Flemish-speaking country. You may find that German grammar is more difficult, but Dutch pronunciation will probably be more difficult for you. I think once you've done the hard work for learning German, then it's fairly easy to pick up Dutch, if you can cope with the pronunciation (which is tough). But if you learn Dutch first, then German grammar will still be a challenge. Another thing is if you are planning on visiting a Dutch-, Flemish- or German-speaking country, think about how good the English is of the people who live there.

English, German and Dutch are Germanic language. French, Italian, Spanish, Portuguese, Romanian, Catalan are Romance language. But English is like a mix of Germanic and Romance. Though grammatically, English is closer with Germanic, English absorbed more than 50% of its modern version from Latin and its children, especially French. Between Dutch and German, Dutch is closer to Indonesian. Indonesian also absorbed many words from Dutch. Examples: D - I asbak - asbak waskom - baskom broeder - bruder boek - buku duit (copper Dutch coins) - duit (uang) kamer - kamar kantoor - kantor bruine bonen soep - sup brenebon kaasstengels - kue kastengel Dutch pronunciation is also similar with old Indonesian pronunciation (Ejaan Van Ophuijsen/Ejaan Lama), like Dutch’s BOEK reads as BUK. Similar with Indonesian’s OENTOEK becomes UNTUK. Hope it helps.

Very difficult question to answer. German and Dutch are similar and also different. I guess if you are more likely to go to Germany, choose German. If you love the Netherlands, learn Dutch!!

must write English, not english

I would say German first. Note that you should say "after mastering English". I recommend German because I'm learning it, whereas I have not been exposed to Dutch. If you want to live, work or visit a German-speaking country, learn German first, and the same for Dutch. I read some great answers here too :)