Differences in language are tied very closely with differences in race. Excluding people based on the language they [don't] speak is simply racism.

Safety is not a concern here. What language you speak has nothing to do with whether you know how to not drown. I'm quite certain that there are plenty of German speakers who blissfully fail to read or otherwise ignore that sign. Finally, the article seems to make it clear that the swimming area is understaffed for the number of bathers visiting. That's a staffing issue.