A really useful post for those whom curiosity killed

 Today by Mistake, I changed the facebook language to Vietnamese.

then I found  this post from Livewire which is a lifesaver 
by Leslie Walker

Good Job!

Updated June 15, 2017

With over 100 languages to choose from, Facebook probably supports your own language so that you can read everything in what's comfortable to you. If you've already changed your Facebook language, you can also read Facebook in English (or any language) again in just a few easy steps.

One of the fun language options on Facebook is Pirate English. Menus and your labels on various pages will change to the pirate lingo, like "sea dogs" and "wenches" in place of "friends." It will definitely look funny to you but you can rest assured that nobody else can see it unless they, too, change their own language settings.

There are even lots of languages you can choose from that most websites don't support, like Tamazight, Malayalam, Kannada, Silesian, and Swahili.

How Do I Change the Language on My Facebook?

It's easy to change the language Facebook displays text in. Either access the Language Settings page through this link and then skip down to Step 4 or follow these steps:

1. Click or tap the arrow on the far right side of the Facebook menu bar, to the right of the Quick Help question mark.
2. Select Settings.
3. Choose the Language tab on the left.
4. On the line that reads "What language do you want to use Facebook in?", select Edit.
5. Choose a language from the drop-down menu.
6. Click or tap Save Changes to apply the new language to Facebook.

Here's another way to change the language on Facebook:

1. Go to your profile's Timeline page.
2. Scroll down enough that the left menu stops scrolling and shows a language section. There are popular languages there that you choose from, like English, Spanish, Dutch and Portuguese.

1. Click one and confirm it with the Change Language button that appears.
2. Another option is to click the plus (+) sign to see all of the supported languages.
3. Select a language from that screen to immediately apply it to your Facebook.

How to Change the Facebook Language Back to English

It might be hard to know how to change your language back to English when all the menus are in a different language that you might not be able to read.

Here's what to do:

1. Go back up to the previous section (the second part) and follow those steps down to Step 2.
2. Choose the one called English.
3. Select the blue button that shows up in the current language.
4. Facebook will translate back to English for you.