The automatic language detection is enabled by default. This option is very helpful if you don't know the language of the word you want to translate.
As you select a word and click the Translate button, the language detection tool analyzes the source word, identifies the language and displays the label "Detected: language".
There may be cases when the language detection tool doesn't recognize the language correctly especially for languages of the same language group (Slavic languages) or to the loan-words.
If you know the language of a word you want to translate, select the known language (i.e. Russian) and lock it by checking the "Lock-in language" checkbox. This will temporarily disable the auto language detection and secure the chosen language.
If you want to permanently disable the automatic language detection, go to Options and set the language direction manually.