The warning is certainly because you're taking the screenshot on a different monitor than your main one. Can't find anything in Tesseract to avoid this... Simplest workaround is to drag your app window to your main monitor and take the screenshot from there. Results should be much better!
By default, Tesseract try to guess language from results, so I guess it should work as is. In the last version of my Alfred script (see https://github.com/nicooprat/alfred-ocr/blob/master/README.md#usage), you can type "OCR [lang]" where "[lang]" is in this list: https://github.com/tesseract-ocr/tesseract/blob/b67ea2c1a70c56053e142a5fb7cc18fb29cdc4b8/src/training/language-specific.sh#L21
I can't help with this one, you should seek help on the Tesseract project: https://github.com/tesseract-ocr/tesseract/issues