How to define a custom ORDER BY order in mySQL

If those are the only three values, then you can use a CASE expression:

ORDER BY `ID`,
         CASE `Language`
         WHEN 'ENU' THEN 1
         WHEN 'JPN' THEN 2
         WHEN 'DAN' THEN 3
         END

(If there could be other values, then you may want to add some extra logic to keep the ordering consistent; for example, you might add ELSE 4 to that CASE expression, and then order by Language itself as the third ordering criterion:

ORDER BY `ID`,
         CASE `Language`
         WHEN 'ENU' THEN 1
         WHEN 'JPN' THEN 2
         WHEN 'DAN' THEN 3
         ELSE 4
         END,
         `Language`

)

MySQL has a handy function called FIELD() which is excellent for tasks like this.

ORDER BY FIELD(Language,'ENU','JPN','DAN'), ID

Note however, that

1. It makes your SQL less portable, as other DBMSs might not have such function

2. When your list of languages (or other values to sort by) gets much longer, it's better to have a separate table with sortorder column for them, and join it to your queries for ordering.