Language Detection tool analyzes the source text, identifies the language and set the correct language from the source language list.
You can set the language direction manually, or mark the "auto detect" checkbox to let the Translator identify the language of the original text.
If you know the language of the source text, do not check the "auto detect" option, and set the language from the drop-down list. This will be your default language for this translation session.
If you want to save the new language settings, go to Translator Options and make necessary changes.