Changing AppStore's language has been one of the most frequent questions asked by iPhone users.
Many people will just tell you to go to Settings and change it from there by choosing the desired language and set it as your phone's. That might change some stuff on the AppStore, but the labels for instance, those won't change.
This tutorial will tell you how to change AppStore's language, including the labels.
Step 1: Go to Settings.
Step 2: Tap Store.
Step 3: Tap your Apple ID and then Sign Out.
Step 4: Now Sign In and choose Create New Apple ID (you won't complete this, don't worry).
Step 5: You'll be asked to choose your store (country/region), choose the country which has the language you want the AppStore to be in.
Step 6: Click Next and then Agree.
Step 7: Fill in the information until you're asked to insert your credit card info.
Step 8: When you reach the credit card info, click Cancel and exit the Create New Apple ID section.
Step 9: Go back to Settings and sign in with your actual Apple ID.
Step 10: Open the AppStore.
Your iPhone's AppStore should now be in the language you wanted, including the labels.
I tried this once and it worked for me.
If it doesn't work for you, leave a comment and I'll try to find another way to do it.