How can I best prepare myself for reenlisting to learn a new language through DLI?

You and your retention NCO might be miscommunicating a bit. It's possible to go to DLI, but not as a 68W. You can go as a Reclass into MOS 35P or 35M. Or if you're an MOS that is language enhanced like SF, CA, PSYOPS, or 35L.
If one of those are the route you choose, bare minimum you will require a DLAB and a TS clearance. When you graduate you will be expected to maintain a 2/2 the read t of your career or lose your new MOS.

Think long and hard about Arabic and it's civilian applications....the Arabic conflict is winding down.....I myself was using cassette tapes....for Arabic.....The future is Mandarin...

Making a nomination for SFC Boyd to be considered wisest NCO in RP. I was a 35PAD for 7 years before I went OCS. I went through DLI in 2002. Everyone who has written before me was correct (except it was 74 weeks when I went through. Guess it’s shorter now). On day one you will not speak English. You will have no clue what they are saying, so it might be a good idea to learn the alphabet and a few of the basic words first. That first few weeks was hard when you sat through 8 hours of Arabic and then you were furiously writing words down using English transliteration to try and look the words up later without knowing much of the alphabet. I don’t know your proficiency with languages. Many people have a DLI vacation because they pick stuff up quick. I was on academic probation the entire time because I didn’t pick it up quick. That meant you went to class until 1600, did PT, had dinner, then went back to the schoolhouse from 1900-2200. I didn’t have a DLI vacation. The good thing is, the schoolhouse taught you above the DLPT, so getting a 2/2 wasn’t that hard.

Here’s the kicker. Those academic DLI vacationers with a head for Arabic? I often outperformed them on mission. The reason is DLI teaches modern standard. Unless you are on an open source intelligence mission, you will never use MSA. The academic bookworms had trouble because they constrained themselves to the grammar and structure rules of MSA making it harder for them to really pick up dialects. Now I hear at DLI they will now allow you to DLPT in the dialects, but unless that dialect is magically the same one you will use at your first unit, I still see my experience applying here. When I was doing the job every day, you still had to maintain your 2/2 in the MSA DLPT even if you were dealing with Iraqi. Now you can DLPT in Iraqi to maintain proficiency.

Last thing. This was one of the biggest reasons why I chose to drop my language later in my career. Though we have made significant headway through the years with a better linguist regulation and better education of our leadership, there are still commanders (especially MICO commanders) who believe maintaining one’s language proficiency rests solely on the Soldier for completion when they get off duty. The Army is the worst military service when it comes to this and CLPMs are constantly working hard to fix this. Bottom line, you need to be prepared to figure out a way to maintain your 2/2 on your own if need be. The average non-linguist does not understand what it takes to maintain your skills to a advanced HS early college level proficiency and they tend to ignore their CLPMs who are their primary advisors in this matter. I never found a good way to maintain my language on my own especially since days typically ended at 1900 and I started having a family.