In German people tend to increase “only” (das einzige). As in, they say something is the “onliest” (das einzigste). It’s usually a good indicator of someone’s education.

In many regions it is common to do comparisons with “as” (wie). As in “My dog is bigger as yours” instead of “My dog is bigger than yours”. The most infuriating thing about this is that most people doing that mistake don’t even acknowledge that it is one. At least people who say “onliest” can be convinced that it is wrong.

Technically not an error but still annoying is to append an apostrophe and an s to a name to indicate the genitive. Like in “Anna’s food is good”. In German that should be written as “Annas Essen ist gut”. But due to many people making the same mistake (I guess also because we’re used to it from English sentences) it has been allowed to use an apostrophe. So in that case I’m just a grumpy old guy.