- stick to one font, use bold/itdlics for emphasizing particular pieces of dialogue
- look in typesetting guides which fonts work better, avoid less-readable ones
- avoid black bars, try to keep the raws' resolution
- get someone who speaks english well and is not the same that translated (in this case yourself) to proofread before you release. As a general rule (in lots of things) the same person doesn't check their own work.
You should reach a reasonable level of quality if you follow the points above.