Hi Webica,
thanks for the feedback. Translator remembers the last two languages you manually selected to translate to, and will choose the one that translates to the language that is different from your input (source) language. In other words Translator will ignore
the shown target language, and switch back to the last selected target language, if a same-langauge translation would result. So you can create the behavior you want by simply selecting target languages twice, as long as you mostly translate between
two languages.
Example:
For the example, always leave source at Auto-Detect.
Choose Spanish as target.
Translate "This is a test".
Choose English as target.
Translate "Esto es una prueba".
Type "This is not a test".
Result: "Esto no es una prueba".
Type: "Esto no es una prueba".
Result: "This is not a test".
Let us know if this helps in your situation,
Chris Wendt
Microsoft Translator