When you start the client up after successful login you will see the 3 buttons in this order (top to bottom):
1. Mulai (Start)
2. Keluar (Exit)
3. Pengaturan Game (Game Settings)
Choose 3 > "Pengaturan Game"
On the left tab is a menu with "Pengaturan Game". It should default to this, and on the right tab you will be presented with the tab heading "Pengaturan terkait Bahasa Regional" (Settings connected to regional language).
Going from top to bottom:
1. Pilih Suara NPC (NPC Voices) - should already be set to English
2. Jenis Watermak (Type of Watermark) - set to Global
3. Filter Obrolan (chat filter) - change to your preference
4. "Pengaturan Bahasa Regional" - Regional language settings. It is default to Bahasa (refers to the entire Bahasa Melayu regional languages, not just Indonesian). Change to English. (This is the most important part for you because you can change all the other settings once you understand what is in the settings).
5. Server: Chat - Kirim & Terima - Server chat settings for receiving & sending - change this to English
6. Finally Accept your choices > press the button "Terapkan" (Apply) followed by "Konfirmasi" (Confirmation)
You will then get a dialog box which says " Silahkan restart klien untuk mengganti bahasa" (Please restart the client to change language).
Click OK .. means OK :P
Hope that helps.