He's been speaking English a long time (despite choosing the dot paper instead of squares) -- as a native English speaker myself those were pretty clear to read. It is interesting that he has apparently abandoned the German letterform handwriting (at least in English).
I assume his students are ver comfortable in English as well.
It has nothing to do with language skills (the parts I can decipher are perfectly decent english); his handwriting is just terrible.
These are the slides for the lecture "Programming Languages and Operating Systems" by PhD student Lucy Amidon. I think they are probably by her, and not by Felleisen. The other lectures with a byline have other distinct styles.
What does dot paper have to do with English? Looks like these were drawn using a mouse.
dot paper is uncommon in the US (where he lives), rather quadrille is the norm. The opposite is true in Germany.
In PL academia (including in the US) many people prefer dot paper for physical research notebooks. When you're mixing together diagrams, prose, inference rules, semantics, code, etc. it's a quite nice and flexible medium.