(Caution: this is just an attempt, I'm not 100% sure on the translation.)
Proposition 5: If a number is part of a number, and another is the same part of another, then the sum is also the same part of the sum that the one is of the one.
Translation: Let x,y,z,x1,y1,z1 be non-zero real numbers. If xy = z, x1y1 = z1, and x/z = x1/z1, then
(x + x1)/(z + z1) = x/z.
Now let's break down some of Euclid's terms. It seems like if we have a number (x) being a part of another number (z), then the term "part" means z/x. For instance, 2 is a part of 8 because 2 is 8/2 = 4 parts of 8. Also, the phrase "the one is of the one" is referring to (x) of (z), where (x) is z/x parts of (z)
Now using the notation above, I will restate the proposition, adding in notes as necessary.
New Proposition 5: If a number (x) is part of a number (z, where xy = z), and another (x1) is the same part of another (y1, where x1y1 = z1 and z/x = z1/x1), then the sum (x + x1) is also the same part of the sum (z + z1) that the one is of the one ((z + z1)/(x + x1) = z/x).
I had to read the proposition like 10 times to get it in my mind. When Euclid says "the one is of the one," it's very confusing and it's not so clear what he is referring to. I actually found the translation by ignoring the one to one part. The propositions 6-11 follow similar wording make similar algebraic claims. So after translating 5, they don't seem so bad. As part of the main daily 2 work, I wrote a proof for this proposition.