What does "Blitzendegen" mean? The term "Blitzendegen" is used by the Dorssian military frequently (see here for an example). It seems to be used as a salute, but it's also used in other context. The term seems to be from German, but its literal meaning in German is "lightning rapier" which doesn't make much sense.

What is the in-universe meaning of "blitzendegen", and when are Dorssian soldiers expected to use it?
No, it's just a modified military salute and the words themselves have no particular meaning or a translation that would make sense.

While a military salute can be used for acknowledgement when receiving orders, it is rather uncommon and makes the person doing so look retarded. Another use of a modified military salute may also be a battle cry. While there exist historic videos from Germany with masses chanting and doing the Nazi salute, flags hanging everywhere and despite fascist songs being sung in the Wehrmacht, usage of symbols and memes resembling those that the Nazis used (and not being very original) are anything but an accurate take on this topic and rather use of artistic license. Producers often use their artistic license in that kind to demonize the enemy and depict dictatorships or fanatics. 

I can't blame them, it still works very well and I found one of my teachers refusing to believe me when I told her that the Schwabacher typeface was abolished in Germany by the Nazis (thus finally putting an end to the Antiqua-Fraktur dispute).

Similar uses of artistic license in Animes:


"Sieg Zeon" in the Universal Century setting of the Mobile Suit Gundam franchise from Sunrise
"Gloria Augusta" in Last Exile -Fam the Silver Wing-
[...]


So what happend here was that they took some well known phrase and replaced one half with something random to have something new and unique. The Dorssian flag is another example of that.