Minor refactoring. Use language's regional translation as key.

Previously, the translation data had the language names translated
in the specific language which is counter intuitive. Because if the
page loads in English and the user clicks on the Change Language
button then they will see their language's name in English which they
might not now.

Signed-off-by: Sudhanshu Gautam <me@sudhanshug.com>

src/components/header.js

@@ -103,6 +103,11 @@ export default function Header() {
                       control={<Radio />}
                       label={t('Turkish')}
                     />
+                    <FormControlLabel
+                      value="es"
+                      control={<Radio />}
+                      label={t('Spanish')}
+                    />
                   </RadioGroup>
                 </FormControl>
               </Paper>