In most cases, Google uses English as an intermediary language, so when you translate a text from Indonesian to Vietnamese, Google translates the text to English and then it translates the result to Vietnamese. You'll get the best results when one of the languages is English, since Google needs a single translation.
"Most state-of-the-art, commercial machine-translation systems in use today have been developed using a rule-based approach, and require a lot of work to define vocabularies and grammars. Our system takes a different approach: we feed the computer billions of words of text, both monolingual text in the target language, and aligned text consisting of examples of human translations between the languages. We then apply statistical learning techniques to build a translation model," explains Google.
One of the advantages of this approach is scalability: if Google finds enough parallel text to create a good translation model for a language, it will be added to Google Translate. When Google licensed Systran's technology, Google Translate was only able to translate between English and French, German, Spanish, Italian and Portuguese, but this has changed when Google developed its own translation technology. Microsoft followed suit and Windows Live Translator switched from Systran to Microsoft's machine translation system.
As Microsoft notes, it's important to keep in mind that "automatic translation enables you to understand the gist of foreign language text, but is no substitute for a professional human translator if fluency is required," at least not yet.