I'm looking for a way to start different qgis instances in different languages simultaneously without changing the settings in qgis.
Is it possible to define the language with a command line parameter or something like that?
Yes, that's possible, just run QGIS from the command line in this way:
qgis --lang en
Then, from another console:
qgis --lang es
However, be aware that using QGIS in one instance might store settings in your operating system that could then eventually interfere with your other instances.