Edit: I looked it up and it is NOT czechia, it IS actually austria. Czechia is 704 km and austria 801km...
Tbh I thought the polish border was more significant aswell. I guess you get that feeling because poland is rather big.
The one source I found that states that czechia-germany is 815 km also states that austria germany is 815km... so something is amiss there..
Wikipedia contradicts itself. If you go to the geography countries you get one number (a: 801 km and c:810km) but if you go to austria-germany and czechia-germany border pages, you get different numbers (a:815/816 (only one that doesnt link to world factbook) and c:815km)
All the numbers but one give world factbook as their (dated, 2012) source but world factbook itself say something different..
The geography of germany page DOES actually have the same numbers as worldfactbook. (Austria 804km and Czechia 701 km)
So austria is consistenly shown to be around 810km (801-816) while czechia differes from 701km to 815 km while All of the different pages give worldfactbook as their source where it is shown as 701km)