Langston Hughes seeks to accomplish two purposes in these poems. First, he is portraying the African American experience at a time when African Americans were not only considered less than human, certainly unworthy of being considered "American", and how those experiences are so pointedly different from his white counterparts (i.e. the instructor). Secondly, in portraying his experience he attempts to demonstrate the basic human similarities between both African Americans and whites despite the obvious disparity in their American experience at the time. Regarding your question, can these poems be relevant to others, not just African Americans? Yes and no. Yes, because all human beings have suffered from some level of ostracism and quite possibly violence simply for being different. However, if you delve beyond that simplistic interpretation of his poetry and see the brutality and historic ostracism of African Americans up to the point Hughes wrote the poems and throughout his lifetime, then it would be difficult to compare that experience to any other ethnic group within the boundaries of the United States.
Hughes' poem, "I, too" is modelled after Walt Whitman's poem "I hear America singing." Hughes was extending Whitman's idea that America can be found in its people by including a particular section, the African-American. Beyond this, however, Hughes was taking Whitman's patriotic and idealistic message and portraying an America still filled with prejudice and inequality. Like "Theme for English B", all humans can attest to feeling misunderstood and feeling discriminated against by others, whether for their color, sex, economic class, fashion sense, musical tastes, and so on. Reading "I, too" should encourage others to create their own "Song of America."
Pertaining to "Theme for English B," read the opening lines which give the teacher's assignment:"Go home and write/a page tonight./And let that page come out of you--/ Then, it will be true." What is your reaction to such an assignment when you hear it? I imagine you feel that your experience is unique to you, that no one can understand it--or maybe that your experience is the same as everyone's and therefore maybe too boring to write about. The speaker insists on both: his experiences will be like those of his instructor in that they are both American, but his will be very different, too, because he is black ("Being me, it will not be white"). Perhaps some people think (regardless of their color) that their family is weird and are afraid to write about that, but it is that weirdness, that sense of difference, that makes them interesting, while still sharing humanity with the rest of us.