use arrow functions where possible

By converting all functions to arrow functions we optimize two things. First we can directly remove some return statements by using implicit return and secondly it allows bundlers to compress mupltiple const declarations into a single const declaration with multiple constants. Example:

const test1 = "test1"
const test2 = "test2"
const test3 = "test3"

Becomes

const test1 = "test1", test2 = "test2", test3 = "test3"

Which saves some bytes that would otherwise be used for the function keyword in the pervious version.

Checklist

[x] only relevant code is changed (make a diff before you submit the PR)
[x] run tests npm run test
[x] tests are included
[x] commit message and code follows the Developer's Certification of Origin

i18next / react-i18next

use arrow functions where possible #1762

Checklist