The biggest thing to keep in mind is that potential host parents, including the ones that eventually pick your child, WILL read it - regardless of what language they speak. It doesn't have a whole lot to do with the application process, other than that.
Again, don't worry about being a good writer or not as they probably won't know if it is well written or not.
Start out with a friendly greeting: Dear Friends, or Dear Parents - don't start with "To Whom it may concern" or something rude like that.
State who your daughter is (first name only) so that they can reference the name on the letter to verify it is the same as the name on the application. "I am (your daughter's name) mother and her father and I are very excited that she has chosen to partake in a student exchange" (say this whether you are, or not - if you aren't than they may be afraid of you trying to be over protective or your daughter or something) Then, say why she is wanting to go on the exchange: "She is excited to learn a new language and culture and can't wait to meet her future host family."
Be sure to include her relationship with other family members, including activities you like to do together - this will give the host family ideas of things they can do with her. "She and her brothers get along when they (do something together, sports or what not), but they also fight with each other, like all siblings do. Her father and I love to take the kids on vacation and do (such and such activities)."
After you state all important relationships, say some things about her. "She loves to cook and play sports and go shopping. (or whatever it is she likes to do.)" This needs to be a pretty large section...the more things you say, the more likely she will get placed with a family that will like her just as much as she likes them. Say a lot of what she doesn't like to do and what she does like to do.
Lastly, state anything that the family definitely needs to know about your daughter. "She is allergic to...". "She is very moody when..." Stuff about her attitude and personality will help the family understand why your daughter acts the way she does.
Most importantly, It is good to say as many good things as you can, but don't lie and don't over exaggerate. Don't say things like "she cooks all the time and is very good at it" if she isn't good, or doesn't cook that often. Don't say she likes to do sports and go outdoors when she really doesn't. Saying things like this will make the family expect something that isn't right and when they "pick" a student that isn't like they were expecting, the exchange may not go over well.