My Support Unit's staff can be divided into two categories: ultra-competent endgame soldiers of rank A++ and up, and a handful of early-game interpreters with ranks as low as C. Those interpreters are dragging down my Support Unit's level, but aside from the Afrikaans guy, I've never been offered an opportunity to replace them.
What options do I have to get better interpreters? If I fire them (can I fire them?), will the "Extract Interpreter" side ops come back? Is there any way to get replacements without losing my existing interpreters, so if the new ones suck worse than the originals, I can keep the originals? I'd imagine that FOB invasions might also be an option, but everyone else's interpreters probably suck too.
(This also applies to the Legendary Gunsmith and anyone else with an important skill you only get one of that I might be forgetting, but my Legendary Gunsmith is A+ rank, so he's not dragging down the R&D Unit as hard.)
Best Answer
Interpreters
You can fire your interpreters and any extra interpreter you may have after a successful FOB mission.
From a handful of FOB invasions of my own, I haven't once found a better interpreter than the ones I already have. In fact, the interpreters were all exactly identical to mine, from name to stats. I therefore assume that there isn't any way to improve your interpreter staff.
However, you do not need to keep your interpreters in Mother Base. As long as they've been assigned to your Support team at least once, you can make use of their interpreter skill even if they're no longer in your base (Thanks @user2357112 for this). Once fired, there is no way to get them back, other than extracting one during an FOB invasion. The only exception being Afrikaans interpreter, one of which can be extracted as many times as necessary by replaying mission 16:
Lingua Franca.Specialists
Specialists are easy to replace. They are found during main missions, and are usually special elimination targets or prisoners. Which specialist you get during which mission, is specified at the top of your iDroid map as you select the mission.
Before you start replaying any of your early missions to get better specialists, however, I'd recommend you take a good look at the repeat missions. Those are exact duplicates of earlier missions, but have restrictions placed on top. These missions usually award you with significantly better staff than their original.
For instance, the mission
Over the Fenceawards you with a Bionics specialist (your main extraction target), who is an R&D specialist ranked somewhere between C and A. However, the[Total Stealth] Over the Fencemission can easily award you the same specialist with an A++ or S rank instead.Keep in mind that the specialists aren't always found at the same location, or found at all. Special extraction targets, like the Bionics specialist in
Over the Fenceor the arms dealer specialized in Missile Guidance inThe War Economy, are guaranteed to always be the same target, however random prisoners aren't guaranteed to always be the specialist you're looking for.For instance, during the
Backup, Back Downmission, I found my Drugs specialist in the Wakh Sind Barracks. While playing the[Extreme] Backup, Back Downmission, I expected the Drugs specialist to be where I found her the last time. And although I did find her exactly where I expected her to be, she wasn't a Drugs specialist this time around. In fact, none of the 6 prisoners I extracted were Drugs specialists. So you may need to replay the same mission several times before you can find the specialist you're looking for.Legendary Gunsmith
As for the Legendary Gunsmith (Master Gunsmith), you can give him the same treatment as your interpreters. You can still customize your weapons even if he's no longer at Mother Base.