Trans Component

Important note
While the Trans component gives you a lot of power by letting you interpolate or translate complex react elements - the truth is - in most cases you won't need it.
As long you have no react nodes you like to be integrated into a translated text (text formatting, like strong, i, ...) or adding some link component - you won't need it - most can be done by using the good old t function.
It does ONLY interpolation. It does not rerender on language change or load any translations needed. Use useTranslation, withTranslation for those cases.
import React from 'react';
import { Trans, useTranslation } from 'react-i18next'
​
function MyComponent() {
  const { t } = useTranslation('myNamespace');
​
  return <Trans t={t}>Hello World</Trans>;
}
Using the t function have a look at i18next documentation:
​essentials​
​interpolation​
​formatting​
​plurals​
...
Samples
Using with react components
So you learned there is no need to use the Trans component everywhere (the plain t function will just do fine in most cases).
This component enables you to nest any react content to be translated as one string. Supports both plural and interpolation.
Let's say you want to create following html output:
Hello Arthur, you have 42 unread messages. Go to messages.
Before: Your react code would have looked something like:
import React from 'react';
​
function MyComponent({ person, messages }) {
  const { name } = person;
  const count = messages.length;
​
  return (
    <div>
      Hello <strong title="this is your name">{name}</strong>, you have {count} unread message(s). <Link to="/msgs">Go to messages</Link>.
    </div>
  );
}
After: With the Trans component just change it to:
import React from 'react';
import { Trans } from 'react-i18next'
​
function MyComponent({ person, messages }) {
  const { name } = person;
  const count = messages.length;
​
  return (
    <Trans i18nKey="userMessagesUnread" count={count}>
      Hello <strong title={t('nameTitle')}>{{name}}</strong>, you have {{count}} unread message. <Link to="/msgs">Go to messages</Link>.
    </Trans>
  );
}
Your en.json (translation strings) will look like:
"userMessagesUnread": "Hello <1>{{name}}</1>, you have {{count}} unread message. <5>Go to message</5>.",
"userMessagesUnread_plural": "Hello <1>{{name}}</1>, you have {{count}} unread messages.  <5>Go to messages</5>.",
​saveMissing will send a valid defaultValue
Alternative usage (v11.6.0)
<Trans
  i18nKey="myKey" // optional -> fallbacks to defaults if not provided
  defaults="hello <italic>beautiful</italic> <bold>{{what}}</bold>" // optional defaultValue
  values={{ what: 'world'}}
  components={{ italic: <i />, bold: <strong /> }}
/>
This format is useful to interpolate a component multiple times. Another advantage is the simpler <> named tags -> which makes guessing indexes a thing of the past.
Existing self-closing HTML tag names are reserved keys. link: <Link />, img: <img src="" />, media: <img src="" /> won't work
Using for <br /> and other simple html elements in translations (v10.4.0)
This was newly added in react-i18next@v10.4.0
Allows elements not having additional attributes like className and only no children (void) or one text child:
<br/>  
<strong>bold</strong>  
<p>some paragraph</p>  
but not:
<i className="icon-gear" />  // no attributes allowed
<strong title="something">bold something</strong> // no attr
<bold>bold<i>italic</i></b> // no inner elements - only strings!
It allows you to have basic html tags inside your translations which will get converted to valid react elements:
<Trans i18nKey="welcomeUser">
  Hello <strong>{{name}}</strong>.
</Trans>
// JSON -> "welcomeUser": "Hello <1>{{name}}</1>.",
<Trans i18nKey="multiline">
  Some newlines <br/> would be <br/> fine
</Trans>
// JSON -> "multiline": "Some newlines <1/> would be <3/> fine"
You can use i18next.options.react to adapt this behaviour:
option
default
description
transSupportBasicHtmlNodes
true
convert eg. <br/> found in translations to a react component of type br
transKeepBasicHtmlNodesFor
['br', 'strong', 'i', 'p']
Which nodes not to convert in defaultValue generation in the Trans component.
Interpolation
You can pass in variables to get interpolated into the translation string by passing down objects in node children containing those key:values.
const person = { name: 'Henry', age: 21 };
const { name, age } = person;
​
<Trans>
  Hello {{ name }}. // <- = {{ name: name }}
</Trans>
// Translation string: "Hello {{name}}"
​
​
<Trans>
  Hello {{ firstname: person.name }}.
</Trans>
// Translation string: "Hello {{firstname}}"
Plural
You will need to pass the count prop:
const messages = ['message one', 'message two'];
​
<Trans i18nKey="newMessages" count={messages.length}>
  You got {{ count: messages.length }} messages.
</Trans>
​
// Translation strings:
// "newMessages": "You got one message."
// "newMessages_plural": "You got {{count}} messages."
Using with lists (v10.5.0)
You can use list.map as children. Mapping dynamic content.
<Trans i18nKey="list_map">
  My dogs are named:
  <ul i18nIsDynamicList>
     {['rupert', 'max'].map(dog => (<li>{dog}</li>))}
  </ul>
</Trans>
// JSON -> "list_map": "My dogs are named: <1></1>"
Setting i18nIsDynamicList on the wrapping element will assert the nodeToString function creating the string for saveMissing will not contain children.
Alternative usage (components array)
Some use cases might be simpler by just passing content as props:
<Trans
  i18nKey="myKey" // optional -> fallbacks to defaults if not provided
  defaults="hello <0>{{what}}</0>" // optional defaultValue
  values={{ what: 'world'}}
  components={[<strong>univers</strong>]}
/>
<0> -> 0 is the index of the component in the components array
Eg. this format is needed when using ICU as translation format as it is not possible to have the needed syntax as children (invalid jsx).
How to get the correct translation string?
Guessing replacement tags (<0></0>) of your component is rather difficult. There are four options to get those translations directly generated by i18next:
use React Developer Tools to inspect the <Trans> component instance and look at the props.children array for array index of the tag in question.
use debug = true in i18next init call and watch your console for the missing key output 
use the saveMissing feature of i18next to get those translations pushed to your backend or handled by a custom missing key handler. 
understand how those numbers get generated from child index:
jsx:
<Trans i18nKey="userMessagesUnread" count={count}>
    Hello <strong title={t('nameTitle')}>{{name}}</strong>, you have {{count}} unread message. <Link to="/msgs">Go to messages</Link>.
</Trans>
results in string:
"Hello <1>{{name}}</1>, you have {{count}} unread message. <5>Go to message</5>."
based on the node tree:
Trans.children = [
  'Hello ',                           // index 0: only a string
  { children: [{ name: 'Jan'  }] },   // index 1: element strong -> child object for interpolation
  ', you have',                       // index 2: only a string
  { count: 10 },                      // index 3: just object for interpolation
  ' unread messages. ',               // index 4
  { children: [ 'Go to messages' ] }, // index 5: element link -> child just a string
  '.'
]
Rules:
child is a string => nothing to wrap just take the string  
child is an object => nothing to do it's used for interpolation  
child is an element: wrap it's children in <i></i> where i is the index of that element position in children and handle it's children with same rules (starting element.children index at 0 again)
Trans props
name
type (default)
description
i18nKey
string (undefined)
is optional. if you prefer to use text as keys you can omit that and the translation will be used as a key. Can contain the used namespace by prepending it key in form 'ns:key'
ns
string (undefined)
namespace to use
t
function (undefined)
t function to use instead of i18next.t
count
integer (undefined)
optional count if you use a plural
tOptions
object (undefined)
optional options you like to pass to t function call (eg. context, postProcessor, ...)
parent
node (undefined)
a component to wrap the content into (default none, can be globally set on i18next.init) -> needed for react < v16
i18n
object (undefined)
i18next instance to use if not provided
defaults
string (undefined)
use this instead of default content in children (useful when using ICU)
values
object (undefined)
interpolation values if not provided in children
components
array[nodes] (undefined)
components to interpolate based on index of tag , ...
i18next.init({
  // ...
  react: {
    // ...
    hashTransKey: function(defaultValue) {
      // return a key based on defaultValue or if you prefer to just remind you should set a key return false and throw an error
    },
    defaultTransParent: 'div', // a valid react element - required before react 16
    transEmptyNodeValue: '', // what to return for empty Trans
    transSupportBasicHtmlNodes: true, // allow <br/> and simple html elements in translations
    transKeepBasicHtmlNodesFor: ['br', 'strong', 'i'], // don't convert to <1></1> if simple react elements
  }
});
Please be aware if you are using React 15 or below, you need to set the defaultTransParent or parent in props.

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*name*	*type (default)*	*description*
`i18nKey`	`string (undefined)`	is optional. if you prefer to use text as keys you can omit that and the translation will be used as a key. Can contain the used namespace by prepending it key in form `'ns:key'`
`ns`	`string (undefined)`	namespace to `use`
`t`	`function (undefined)`	`t` function to use instead of i18next.t
`count`	`integer (undefined)`	optional count if you use a plural
`tOptions`	`object (undefined)`	optional options you like to pass to `t` function call (eg. `context`, `postProcessor`, ...)
`parent`	`node (undefined)`	a component to wrap the content into (default none, can be globally set on i18next.init) -> needed for react < v16
`i18n`	`object (undefined)`	i18next instance to use if not provided
`defaults`	`string (undefined)`	use this instead of default content in children (useful when using ICU)
`values`	`object (undefined)`	interpolation values if not provided in children
`components`	`array[nodes] (undefined)`	components to interpolate based on index of tag , ...

option	default	description
`transSupportBasicHtmlNodes`	`true`	convert eg. <br/> found in translations to a react component of type br
`transKeepBasicHtmlNodesFor`	`['br', 'strong', 'i', 'p']`	Which nodes not to convert in defaultValue generation in the Trans component.