iPhone 5 translating to French when I am writing message in English

Switch to another keyboard

While you're using an app, you can switch to a different keyboard, like a third-party keyboard or a language keyboard. Follow these steps:

1. Touch and hold  or .
2. Select the keyboard you want to use. 

To arrange the order of your keyboards:

1. Go to Settings > General > Keyboard and tap Keyboards.
2. Tap Edit.
3. Touch and hold  to move the keyboard.
4. Tap Done.

Sometimes, third-party keyboards aren't available for one of these reasons:

• If the developer of the app you’re using doesn’t allow third-party keyboards.
• If you type in a secure text field, like a password entry that shows typed characters as dots instead of letters and numbers.
• If you use a number pad instead of a standard keyboard.

Instead, you’ll see the default keyboard until you finish typing.

About the keyboards settings on your iPhone, iPad, and iPod touch ...

https://support.apple.com/en-us/HT202178