To see what language has been detected in the Pop-up Bubble application, choose ANY language from the source language list.
The automatic language detection will analyze the source text, and the "detect language" will be switched to the DETECTED language.
You need to make sure that the "Detect source language automatically" checkbox is enabled in the Pop-up Bubble Options.
If you want to save this language setting for future translations, mark the checkbox next to the displayed language to the lock the source language. This will temporarily disable the automatic language detection.
If you want to keep this language as a default language, go to Options and make desired changes.
This functionality is effective in the Pop-up Bubble Translator in all translation extensions: Chrome, Firefox, Opera, Yandex.