The first stanza establishes the poem's setting, as well as its main character: "you." These opening fives lines begin to sketch a particular immigrant story, asking "you" to adopt this "Foreign" person's perspective and "Imagine" their experience.
In the scenario the poem lays out, "you" have been living in the same "strange, dark" foreign city "for twenty years." It seems you have struggled financially, possibly as a result of discrimination, because even after two decades, you're still living among "dismal dwellings on the east side" of the city. The poem never specifies a geographical location or time period, so it's hard to know exactly what "the east side" holds, but the general idea is that it's a poorer or less desirable part of town. It may be an area that residents have a hard time escaping due to exclusion and lack of opportunity elsewhere.
However, it doesn't seem to be a community of immigrants like yourself, since you're acutely aware of your "foreign[ness]" in this environment. In the stairwell of your building, "you hear / your foreign accent echo," as if the walls are throwing your difference back in your face. Even after two decades, you haven't fully adjusted to your adopted country's culture: "You think / in a language of your own and talk in theirs." You feel divided against yourself—and divided from the native residents, too: the collective "th[em]" who seem like foreigners to you.
This opening establishes the poem's form: cinquain stanzas with lines of roughly even length, but no meter or rhyme. Frequent enjambment and caesuras give the lines a halting rhythm, and the diction is generally plain. These effects help evoke the difficulty of speaking in a second language, guiding the reader into the mindset of the "you" the poem describes. If the language were smoothly rhythmic, elaborately rhymed, and full of fancy vocabulary, it wouldn't fit the character or subject matter well. Moreover, cinquain stanzas are relatively uncommon in English poetry, so they have the potential to seem a bit "foreign."
At the same time, touches of alliteration (e.g., "dismal dwellings"), internal rhyme ("stairs"/"theirs"), and imperfect or slant rhyme ("years"/"hear," "years"/"theirs") suggest some effort to organize the poem's language—perhaps reflecting the way "you" have tried to master a foreign tongue. The stanza pattern and roughly even line lengths add some consistency to the free verse, perhaps hinting that "you" aren't truly free in this setting.