。
Incidentally, I'm completely hopeless at English.
Literal
Incidentally I [topic-は] English [subj-が] totally no-good [explanation-なんです].
ちなみに 'incidentally / by the way / for what it's worth' is a discourse marker that introduces a tangential fact. からきし is a casual intensifying adverb meaning 'completely / not at all,' almost always in negative contexts: からきし駄目 'totally useless,' からきし分からない 'don't understand a thing.' なんです is the explanatory んです in its polite form, softening the admission.