I was reading a bit about the Canadians at the Second Battle of Ypres (notably their last stand in the mustard gas) and also the United States Marines at the Belleau Wood and it stood out to me that you always have first hand accounts of them and their allies and you also get some highly propagandized versions of what the they thought the enemy thought of them, but I don't think I've ever read much from the German perspective in these instances.
Are there any good sources for German reactions to these two moments, or any other major act of bravery that is still remembered today? (As another example, the Brécourt Manor Assault)
Sorry that the question is very broad, but I'd be curious about answers about any well known event, really. Far too many times have I read about them and the only note about the enemy is how awesome and cool and scary they thought 'we' were.
While I can tell you very little about the German reaction to the Canadian Troops at the Second Battle of Ypres in particular I can tell you about the German opinion of the Canadian Corps at Vimy Ridge because following that, the German high command actually conducted a full investigation. However, the results of the inquiry reveal that the Germans thought nothing special of the Canadian troops in terms of the fighting quality of the Canadian soldier. To them, the Canadians were just another kind of Tommies from another part of the British Empire.
The inquiry into Vimy Ridge was conducted not because the Canadian Corps were considered to have done remarkably well. In fact, the Germans considered Vimy Ridge, a battle often hailed as the great victory of the Canadian Corps, nothing more than inconclusive because no breakthrough followed. In was just another mile lost at the cost of 20 000 men.
The reason they launched an inquiry was because the defenses at Vimy Ridge should have held and repulsed the attack. They did not. The conclusions of the inquiry are the German frontline commanders ignored warnings about an imminent attack, kept reserves too far back to launch an effective counter-attack and relied too heavily on fortified strong points which were vulnerable to encirclement and caputre unless relieved by counter-attack which as previously stated were too slow on account of reserves being unavailable.
The Germans did keep close tabs on the Canadian Corps though, their movements were tracked meticulously. This was not because they were particularly concerned with the Canadians as such but because they too knew that the Canadian Corps was always kept together as a unit which meant that wherever you saw one Canadian, it was fairly safe to assume there were 99 999 more and 100 000 of any nations troops is something you want to keep tabs on.
Generally speaking what we do know about German opinions of foreign troops is based not on single heroic last stands but on overall preformance. We know the veterans from the eastern front had a great deal of respect for the Siberian Rifles and Guards Rifle divisions for example. Not because of a single action but because these were well trained, well equipped units that generally preformed well. The same goes for veterans of the Italian campaign who had great respect for gorkhas. In broad terms most German soldiers were aware of the strengths and weaknesses of their opponents. They quickly learned to respect allied air superiority, the volume and precision of allied artillery and so on but these things are based on shared collective experience, not single actions.
Some cases, such as German soldiers calling US Mariens 'Teufelshunde' or "Devil Dogs" are almost certainly fabrications, Teufelshunde is not a word that was in use in German at the time, or any time really. The correct term would have been Höllenhund(en) which means roughly Hellhound. A pet theory of mine has always been that it's some kind of mistranslation of the word Hündin which, somewhat less flatteringly, means 'female dog' or... well... bitch.
"The German Army at Vimy Ridge" Hayes, G. 2007
"Constructing Memory: The Vimy Memorial" Pierce, J. 1992
"What Jerry Thinks of Us... and Himself" from Intelligence Bulletin, Dec. 1944 (WW2 opinions)