Trivially easy for a human, maybe. In most cases it's pretty straightforward, but there are a few notoriously tricky language pairs that any automated solution

Trivially easy for a human, maybe. In most cases it's pretty straightforward, but there are a few notoriously tricky language pairs that any automated solution is going to have trouble with. Most notably Norwegian and Danish (and Swedish to a lesser extent), and Czech and Slovakian. There are also some tricky cases in the Iberian area where some dialects of Spanish are quite similar to Portuguese, and in the Balkans Croat, Serbian and Bosnian are basically identical as well, although in some cases they can be distinguished based on the writing system used.