Sadly you’re right. I also saw only German postcrossers with this. Surely, there may be also French, Russian, Spanish … with profiles in just their own language but so far I stumpled just on German profiles in German language.
But I gave up to mention that problem. I either got unpolite or no answer (and when I remember those profiles some time later and had a look they were still written only in German, so …).
I tried to find that point in the rules but as far as I remember there’s no really clear point that it is need to be written in English.
I only found this:
“English is probably the safest option.” at → Help and F.A.Q → Sending postcards → What language should I use?
So this:
is really a good idea I think.