Being that it is difficult to understand Selichos in its original form, would it be better to read all or part of it in English or in a language which is easier understood? Would there be any difference between fast days and the Selichos before Rosh Hashana?
Slichos (both of before Rosh Hashana and of a Taanis) is like any other part of Davening which one should understand what he is saying.
It is preferred that one should learn the meaning of the words of Slichos so he would understand it when he Davens in Loshon Hakodesh.
Alternatively, he can use a Selichos with translation on the side and pause before each paragraph and read with his eyes and then read the paragraph in Loshon Hakodesh after.
On the other hand, if for whatever reason one does not have an understanding of the words of Selichos, he should say the sections that he does not understand in English or any other language that he understands until he learns the meaning of the words, as Davening without Kavana is of little value.
It was the common custom for women who did not understand the words of Selichos to say it in the language that they understand, as when they say it in their own language it brings more concentration and feeling.
But men should say it in Loshon Hakodesh.
See here as well:
I am an educator at a school where many students can’t read well. We have attempted various methods to get OUR Bochurim (mostly Chabad) involved in Davening including giving reward for being Chazan. Is it permitted for one that is not fluent in Hebrew to be Chazan in English? If yes do any parts have to be in Loshon Kodesh?
שוע”ר או”ח קא, ה. ובכ״מ שסליחות מתאים לכל חלקי התפילה. וראה גם סליחות עם לקוטי טעמים ומנהגים לכ”ק אדמו”ר שליט”א. ולהעיר גם מזה שאומרים קדיש תתקבל לאחריו. וכ”ה בפשטות דאין חילוק לענין זה בין תפילה לסליחות, ובכלל תפילה היא.
וראה שו”ת נחלת ישראל שאלה טו, שכתב שמנהג הנשים שדמעותיהן מצויות ושערי דמעות לא ננעלו לומר סליחות ותהילים בל”א, משא”כ אנשים. עיי”ש.