There's something weird going on with Italian localization.
Everything looks correct wrt localization, but it's stubbornly using the English translation instead. I've deep-checked the entire localization chain, but I'm failing to find the cause.

Seen on 91.0.1 (64 bit), Win10.

STR

1. Download Italian version of TB 91: https://www.thunderbird.net/en-US/thunderbird/all/#I
2. Install TB without setting up an account.
3. Right-click on tab bar and show main menu bar.
4. From main menu, choose Strumenti (Tools)
5. Check out Italian menu item equivalents for Addons and Themes or Preferences

Actual result

• Menu items are showing in English: Addons and Themes and Preferences
• Looks like more Fluent translations are affected.

Expected

• Menu items should show in Italian:
  • 'Componenti aggiuntivi e temi' (for Addons and Themes)
  • Preferenze (for Preferences)

Code analysis:

Note: I extracted omni.ja of Italian local install to check the actual shipped files in addition to comm-central. Both of the test strings are contained in the Fluent file menubar.ftl.

1. menubar.ftl is correctly linked in xhtml:

• https://searchfox.org/comm-central/rev/93487d295e3d3365c3082104ea980c7a0dfceeef/mail/base/content/messenger.xhtml#79
• local messenger.xhtml also has the link in line 118 (because DTD links are inserted from messenger-doctype.inc.dtd before)
  <link rel="localization" href="messenger/menubar.ftl" />

2. menubar.ftl exists locally as \localization\it\messenger\menubar.ftl

• The bug might be somewhere here! Looks like the entire file is not read.

3. data-l10n-id looks correctly set (also checked locally in messenger.xhtml):
   https://searchfox.org/comm-central/rev/93487d295e3d3365c3082104ea980c7a0dfceeef/mail/base/content/mainNavigationToolbox.inc.xhtml#1049-1051

        <menuitem id="addonsManager"
                  data-l10n-id="menu-addons-and-themes"
                  oncommand="openAddonsMgr();"/>

4. localized menubar.ftl contains the correct translations with correct Fluent IDs:
   https://searchfox.org/l10n/rev/b8f424f45e760a930175ab37cd29ffd5d8bb2f88/it/mail/messenger/menubar.ftl#20-22

menu-addons-and-themes =
    .label = Componenti aggiuntivi e temi
    .accesskey = C

Also checked in the shipped file.

I have installed all of the following localized Thunderbird installs and then checked if the menuitem label of Tools > Add-ons and Themes was correctly localized:

German (not on pontoon) - success
French (on pontoon) - success
Italian (on pontoon) - fail

I conclude that pontoon (or not) doesn't seem to play a role for the Italian problem.
Italian gets the en-US fallback translations from the localization/en-US folder inside omni.ja.
Accordingly, removing the en-US folder makes the menuitems disappear as the fallback translations are gone (easily tested via unpacking omni.ja into Thunderbird program folder and renaming it to omni.ja.disabled so that the flat files will be used).
However (and not really surprising), removing en-US still won't force the existing Italian translations to be used.

You're loading 3 calendar files in this view?
https://searchfox.org/comm-central/rev/93487d295e3d3365c3082104ea980c7a0dfceeef/mail/base/content/messenger.xhtml#83-85

AFAIK nobody told localizers that Calendar needs to be localized to ship Thunderbird, and Italian doesn't have one of those 3 files.

Maybe that's causing the whole thing to fall back to English? But I'd expect more strings to be in English at that point.

(In reply to Francesco Lodolo [:flod] from comment #2)

Maybe that's causing the whole thing to fall back to English? But I'd expect more at that point.

I've tried to push empty files for Italian. If someone could check (on beta?) if that solves the issue, it would be great
https://hg.mozilla.org/l10n-central/it/rev/fa53ae64c63866e64b7d8986a98ff9f1896031e9

At that point, I expect a lot of locales to have a similar issue. I don't even know which locales have Thunderbird enabled in Pontoon, and not Lightning.

(In reply to Francesco Lodolo [:flod] from comment #3)

I don't even know which locales have Thunderbird enabled in Pontoon, and not Lightning.

Not many, see bug 1647990. Since calendar (lightning) is integrated in Thunderbird core since 78 it should be none, but....

I guess this problem is bug 1464156.

(In reply to Magnus Melin [:mkmelin] from comment #4)

(In reply to Francesco Lodolo [:flod] from comment #3)

I don't even know which locales have Thunderbird enabled in Pontoon, and not Lightning.

Not many, see bug 1647990. Since calendar (lightning) is integrated in Thunderbird core since 78 it should be none, but....

I think that bug was about something else?

With that said, the answer it two: ast, km.

Test beta build with the updated empty files from comment 3:
https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/Qf1av0HOT7iJGv6GaRuavg/runs/0/artifacts/public/build/it/target.tar.bz2

Looks much better.

IIUC, the path forward looks like:

• Push empty files for other locales that are missing them. Flod has indicated that he should be able to do this by EOD tomorrow (Aug 19).
• Run a l10n-bump job on comm-beta followed by a build with l10n repacks to verify.
• Update comm-esr91 l10n-changesets.json (essentially a l10n-bump) with comm-beta's file.

(In reply to Rob Lemley [:rjl] from comment #6)

• Push empty files for other locales that are missing them. Flod has indicated that he should be able to do this by EOD tomorrow (Aug 19).

This is now done.

This is now resolved with the work done in comment 6, right?

(In reply to Magnus Melin [:mkmelin] from comment #8)

This is now resolved with the work done in comment 6, right?

It should I think, in theory, be resolved with 91.0.3. I downloaded 91.0.3 Italian and it looks OK.

However, I updated 91.0.1 Spanish to 91.0.3 and it wasn't 100% good. But maybe that's an issue of incomplete localization?
Not localized:

• account setup initial screen
• add-ons and themes
• preferences search field has "Find in Preferences"

Ok, let's close this then. Resolved by comment 6.

91.0.3 only had the update for Italian.
https://hg.mozilla.org/releases/comm-esr91/rev/f134c939baab6193527545e8f40229aee95d5abf

Gets the remaining files that were missing plus translations that may have happened since the last update.

(In reply to Rob Lemley [:rjl] from comment #11)

91.0.3 only had the update for Italian.

I don't think this is accurate? I don't think someone manually copied the changeset hash for Italian, so you picked up all changes.

Note that Spanish localization is not maintained in Pontoon, per localizer request (Ricardo). So, there's no easy way to check its status now that ELMO has been taken down.

tl;dr: it's quite behind.

es-ES:
errors           25
missing        1237
missing_w      8184
obsolete        140
changed       14718
changed_w     74838
unchanged      1231
unchanged_w    1528
keys           2504
85% of entries changed

(In reply to Francesco Lodolo [:flod] from comment #12)

(In reply to Rob Lemley [:rjl] from comment #11)

91.0.3 only had the update for Italian.

I don't think this is accurate? I don't think someone manually copied the changeset hash for Italian, so you picked up all changes.

Correct, I copied the latest pinnings from beta for 91.0.3. That was on August 18. You added the missing .ftl files for other languages on the 19th in comment 7.