Thanks for your reply. I think you can reproduce it as it is very simply. To answer your question before, yes I tried this scanner as several others too. I'm finally using this you've linked – a good scanner, thanks.
Back to topic. This app is using the language of the mobile after using the barcode scanner. No matter which language you have set within this app. So if your mobile's language is GERMAN and you set english in this app, the app is using german after the use of the scanner although app settings still say "language > english". If your mobile's language is SPANISH and you set english in your app, the app is using spanish after the use of the scanner although app settings still say "language > english". This is tested. If your mobile's language is ENGLISH and you set english in your app, the app is (of course) using english after the use of the scanner (which is finally the only solution I have – set my mobile to english).