Similarly installed UP5 on LMDE 64 bit and have same text problem as described. Also have problem in my 'Gramps' genealogy program on same PC in respect of garbled text in 'Ancestry' tree boxes. None of these thing happened in LMDE whilst on UP4.
Fortunately I have a cloned drive with original UP4 version of LMDE made just prior to applying UP5, so have a backup I can fall back to if need be, but would hope that some solution turns up eventually.
I have tried:
 Upgraded versions of LMDE 64 bit on two other PC's, and problems appear exactly the same on separate LMDE installations and on separate PC's.
 Running Gramps on two other LMDE PC's, but still not right.
 Running Gramps on Mint 13 Maya (Cinnamon & Mate) and on Mint KDE. NO problem so it is not a Gramps bug or corrupted Gramps file.
 Installed proprietary NVIDIA Geforce drivers and removed Nouveau (on spare PC) and checked that I am running on NVIDIA drivers. This has not made any difference whatsoever to the garbled text problem but might have marginally better font rendering (but this might be hopeful imagination on my part).
 Installed (substituted) the Ubuntu texts in etc/fonts, and text rendering is better/prettier, but still random garbled text problem.
 Various combinations of desktop 'themes', and tweaked the fonts settings.
Despite above, still have same problem with garbled text elsewhere in terminal, on desktop, windows titles, nautilus file screens, and it happens in both MATE & Cinnamon desktop versions of LMDE regardless.
From some of other forum topics re upgrading woes, I think the problem is a bit widespread.
Very much a LMNewb myself so can't claim to be any sort of expert, but I am reasonably confident that (by elimination), it is not hardware (PC or graphics card) or driver issue. Also not down to a botched upgrade as I fortunately had no problems (apart from download speed) on any installation upgraded (did 4 PC's). Hopefully the elimination of things might help, particularly with your suggestion of changing to NVIDA drivers.
Hopefully Clem & team + other cleverer LMDE users will come up with some solutions in due course (fingers crossed), but will live with this annoying problem till then as really like and am committed to LMDE.
Main PC = Generic desktop PC with Intel i7 firstname.lastname@example.orgGhz, 12 Gb DDR3 ram, NVIDIA GT215 (GeForce GT 240), 600 Gb Western digital 'Velociraptor' main drive.