I was scouring the game for bugs and errors, and noticed several untranslated bits in the saves menu, which is quite weird since I'm pretty sure they were fine in DQ classic.

It turns out that's because some lines are actually missing from the Italian files, in fact, they are missing from all non-English files, which means that at the moment they can't be translated at all. Not only that, but the Italian version of core.tsv (the file that contains most of the UI) is longer (1158 lines) than the English version (1150 lines) despite actually missing the aforementioned parts. As for the other languages, their files are generally shorter than the English ones, so they too are definitely missing something.

Has anyone noticed similar issues with the UI, cutscenes etc?

In the French game:
. the books are not translated (almost all in English)
. the UI is quite good, it has few words still in English
. the films are well translated, but there is still very few errors
I'll report those errors here.

'Books' as in the journal? Or the equipment for Azra? There were a couple of bugs that may have caused this, but they should be fixed in patch 2.08, which should come out this week.

I mean the journal, is not translated.
The books of Azra are like the UI, translated almost.

That's weird, it was translated in DQ classic. Maybe they ported over an old version of the file. I'll post a report on github if it's not fixed with the next patch.

^
It's still a problem in 2.0.8. The journal is in English simply because the file is written in English, so either the French translator is starting from scratch once again (very unlikely) or someone uploaded the wrong file.

I'll post a report on github tomorrow if ERISS doesn't beat me to it. Also don't rely on labels too much, only a few people can add them and not every issue gets tagged correctly. When you want to report a bug it's a good idea to do a manual search first.

Can someone confirm if the cutscene speeches with black background are always in English in your playthrough?

Edit: nevermind this, I think I figured it out.

The journal should be fixed in the next patch. How's the UI in 2.0.8? Anything else you wish to report?

In descriptions of skills, there are still "Dmg", "Cooldown", "Target" (->Dégâts, Intervalle, Cible),
and in Accessibility options "Failure Penalty", "Hero Mode", and "Restore Defaults" (->Pénalités de Défaite, Mode Héroïque, Réinitialiser).
This is in written "DQ1 2.0.8 c" game.

This is a known bug, the devs will look into it.

This is a different bug. Not sure when it's going to get fixed, that's up to the French translator. I think I can fix the things you mentioned though, so if you care to try:

-Save the attached file
-rename it to core.tsv
-put it in Defender's Quest/assets/locales/fr-FR (it needs to overwrite the existing file, so you might want to make a backup first)
-restart the game

It works. But I see their tooltip (mouse popup) are not translated.
Please remove my 's': Pénalité de Défaite.

I noticed, and there's also that 'current save file' line, and there's probably some more stuff, but I don't speak French...

Anybody with 5 minutes free could play the last mission and check if the names of the civilian NPCs are localized?

Also, when you beat a mission on advanced difficulty, do your XP\scrap bonus rewards appear in English?

The Exit? There are only combattants in this mission (and in others too, usually).
The waves names of enemies seem okay.
In the result window after the mission? Seems good (in french for me).