The 5" screen has a resolution of 600x800, just like the screen on the Kobo Mini, but that's about the only thing these ereader have in common. The PocketBook 515 gave up a lot of hardware in order to hit its price point.
Due to the low price, this ereader doesn't have a microSD card slot, Wifi, a touchscreen, or audio. It truly as limited as the Trekstor Pyrus Mini, the 4.3" ereader which coincidentally costs about the same as the PocketBook 515.
But in spite of its hardware limitations, the PocketBook 515 is still moderately capable. It supports a broad selection of ebook and office doc formats including Epub, PDF, FB2, CHM. DJVU, RTF, txt, html, DOC, and Docx. In addition to letting you read ebooks the 515 also includes apps for a calculator, calendar, clock, and a few games (Snake, Sudoku, Chess).
There's no word yet on whether this ereader will be available in other markets, but even if it does show up in Europe I am not sure I would recommend it.
The Pocketbook 5151 is priced at the equivalent of €69, and I can see that the Kobo Mini is being sold by a number of retails for €59. The Kobo Mini has a touchscreen and Wifi, and based on a simple comparison of the specs it is a much better buy.
On a related note, the Mini is also being sold in the US for $40 and in the UK for £29, making it the cheapest leading ereader anywhere at the moment.