In"52" Derek Walcott's lines "because we serve English, like a two-headed sentry/guarding its borders? No language is...

In my opinion, there is both personification and a caesura
in these lines from Walcott's poem.

I think that there is a
caesura in the lines you cite.  The caesura comes when there is the question mark after
borders.  At this point, there is a break that is caused by the rhythm of speaking, not
by the meter of the poem.

I think you can also argue that
there is personification here.  I think that the idea that English has borders is a
personification because a language does not have borders.  However, you can argue this
is not personification because English is being compared to a country, not a
person.

So if it has to be one or the other, I'd go with
caesura.