Some examples of the awkward grammar and phrasing:
- Each type of weapon have their own little gimmick.
Change it to "Each type of weapon has its own gimmick."
- A spear that is made to make easier for its user to secure hits on its target.
This could be a little less wordy, like, "A highly-accurate spear," or "A precision-crafted spear that is highly accurate."
- A small shield planned for duels that increases the strength of the guard.
It's not clear here whether you're just adding flavor (by indicating that this shield is used by guards), or if this has a mechanical impact (increasing the strength of the Guard skill, or increasing the user's Strength stat). Depending on which you mean, one of these examples would work better: "A small shield used by guards in training," or "A small shield used in duels that increases the user's Strength."
- There is people who try to escape the country by Turtle Forest because it is the closest to the border. So the Kingdom send patrols check there often for heretics.
There are many ways to approach this quote. It seems to me the point of this is to build your world while explaining to the player why they are going on this mission. The best way to reword this will depend on how you want this NPC to feel about the situation. Say the NPC is disapproves of people fleeing the country. His text could look like this: "You're here to patrol the Turtle Forest? Great! Those heretics keep stealing my food as they flee the country! Be sure to bring them to justice!" Or maybe he sympathizes with the escapees: "Another king's guard is here to patrol the Turtle Forest? Maybe the king should ask himself why so many of his citizens are trying to flee the kingdom instead!" Both are a little less awkwardly-worded while making the dialog more natural-sounding.
To be fair, I have seen many native-English speakers write some pretty awkward things, too, so you are by no means terrible; I just appreciate it when game designers polish their text so it isn't confusing or clunky.