It is written in Tanya Rabati that if one is reciting Selihot without a Minyan, one should not recite those sections which are in Aramaic. The reason given is that the angels do not comprehend Aramaic, and only in a Minyan is Hashem Himself present and can understand all languages. This is the opinion of several rabbis as well, including Rav Ovadia Yosef, and some communities follow this practice.
Nevertheless, it is written in Teshuvot HaGeonim that even is there no Minyan, the Aramaic sections may be recited. Indeed, Rabbi Meir Mazuz explains that the the custom in Tunisia and other North African communities was to recite the Aramaic sections of Selihot even without a Minyan. Rabbi Ben Zion Abba Shaul agrees with this position and explains that even if the angels may not comprehend Aramaic, there is no disadvantage to saying those sections and perhaps those prayers will ascend to Hashem directly.
Interestingly, the Pri Megadim and Rabbi Shlomo Zalman Auerbach (Halichot Shlomo, Hilchot Tefila, 17:12) both say that even for those communities that do not recite the Aramaic sections without a Minyan, if the Selihot commenced with a Minyan and some people left, the remaining congregants may continue reciting those sections
Summary: The Aramaic sections may be recited without a Minyan. For those communities that do not follow this practice, if the Selihot began with a Minyan and people left, even they would be able to continue with the Aramaic sections.