Changing region from Vietnam to English in Apple Store app

If you are eligible for a region change, see this on how to to that

1. Open the Settings app. 
2. Tap your name, then tap Media & Purchases.
3. Tap View Account. You may be asked to sign in.
4. Tap Country/Region.
5. Select your new country or region.
6. Tap Change Country or Region.
7. Tap your new country or region, then review the terms and conditions.
8. Tap Agree in the top right-hand corner, then tap Agree again to confirm.
9. Select a payment method and enter your new payment information and billing address, then tap Next. You must enter a valid payment method for your new country or region.*

Make sure that you meet all of the requirements in order to change your country or region first

• Spend any balance remaining on your Apple Account. If you’re not sure whether you have a balance or not, check your Apple Account balance.
• Cancel any subscriptions that block a country or region change, and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, film rentals or Season Passes to end, and wait for any pending store credit refunds to be processed. You can cancel a pre-order.
• Make sure you have a valid payment method for your new country or region. See what payment methods you can use.
• Re-download your apps, music, films, TV programmes and books on any devices you might use to play or read them in the future. Some types of content may not be available in your new country or region. Find out what types of content and services are available in your new country or region.
• After you’ve updated your Apple Account billing address and payment method, you can upgrade your iCloud+ plan using the pricing in your new country or region.

Hello,

This may be a region setting issue and not an actual change in country unless you truly went through all the steps necessary do that (and will have to reverse them to change back). This can sometimes happen if you find a link on the web that points to an app or a media item in another country's iTunes Store. Here's how you can correct this:

Sign out of the iTunes Store [ Sign in to your Apple Account - Apple Support ] and sign in again to get the iTunes Store region to match the region to which your Apple ID country is set, as described in this post: apple id is only valid in us store - Apple Community

1. Go to: Settings > Account > Media & Purchases > View Account > Country/Region

2. Make sure it is set to your normal country.

3. In the "Media & Purchases" menu, sign out and then sign in again.

The above is a simplified version of that in the support document 'Change your Apple ID country or region' ( Change your Apple Account country or region - Apple Support ) except you shouldn't need to do the extra steps involved as when you have done a true country change. IF you did actually change country, that article tells you what you need to do to change it back.