Yana, can you ask the translate team if they no longer support Luxembourgish and Samoan? According to https://bugs.chromium.org/p/chromium/issues/detail?id=585313 they should be supported.

From ftang@:
I look at the code. I suspect what happen is after we fetch the list which contains the sm and lb, it call IsLocaleNameTranslated(() in https://cs.chromium.org/chromium/src/ui/base/l10n/l10n_util.cc but it report sm and lb as false.

When it return false it report the histogram in Finch 
    if (!l10n_util::IsLocaleNameTranslated(lang.c_str(), locale)) {
      TranslateBrowserMetrics::ReportUndisplayableLanguage(lang);
      continue;
    }
https://cs.chromium.org/chromium/src/components/translate/core/browser/translate_language_list.cc?dr=C&l=328

and you can see such report below

https://uma.googleplex.com/p/chrome/histograms?endDate=20170613&dayCount=1&histograms=Translate.UndisplayableLanguage&fixupData=true&showMax=true&filters=platform%2Cone_of%2CA%7CC%7Ci%7CL%7CM%7CT%7CW%2Cchannel%2Ceq%2C4%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

so the issue is not the server does not return 'sm' and 'lb' but l10n_util::IsLocaleNameTranslated return false on that two values (also  "hmn")

So the answer is that the translation server supports these languages, but Chrome does not have the required text translated into these languages to allow them to be supported for translation (also note Hmong is not supported as well). If we want to support these languages we will need to get text translation work done.

This is due to ICU data customization. 

This is the list of languages supported by Translate: 

https://translate.google.com/translate_a/l?client=chrome

I have to update the ICU data to have all the languages in the list covered ('covered' meaning that their names - native and in each of Chrome's UI languages - are included). 

Ok. I'll update the ICU data. There is another bug about a similar issue (v8's Intl support for various es-* variants) that requires the ICU data update. 

See https://cs.chromium.org/chromium/src/third_party/icu/scripts/accept_lang.list
and https://cs.chromium.org/chromium/src/third_party/icu/scripts/trim_data.sh?rcl=dfa798fe694702b43a3debc3290761f22b1acaf8&l=26  (fliter_display_language_name )

How about 'hmn' (Hmong: well, it's a macro language)?  It's also listed in the list in comment 4 but missing in Chrome's ICU data. 

Issue 740577 has been merged into this issue.

Looks like it's 4 total languages missing: 
Hmong
Javanese
Luxembourgish
Samoan

Check out reg's bug 740577 that shows how this creates user issues

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/deps/icu.git/+/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50

commit 01ab158fd9c6ce1f1444f616f06fd4fb409bdb50
Author: Jungshik Shin <jshin@chromium.org>
Date: Mon Jul 17 23:36:25 2017

Add the display names of 4 languages

They're supported by Google Translate and their names need to be shown
in the Translate UI.

Hmong (hmn)
Javanese (jv)
Luxembourgish (lb)
Samoan (sm)

The size impact for desktop Chrome is about 3kB.
( 10,175,056 => 10,178,576)

BUG= 733398 
TEST=See the bug
TBR=yyushkina@chromium.org

Change-Id: I9ca232343847a9a95c1dabacb6336bd43ba39849
Reviewed-on: https://chromium-review.googlesource.com/574771
Reviewed-by: Jungshik Shin <jshin@chromium.org>

[modify] https://crrev.com/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50/android/icudtl.dat
[modify] https://crrev.com/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50/common/icudtb.dat
[modify] https://crrev.com/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50/common/icudtl.dat
[modify] https://crrev.com/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50/ios/icudtl.dat
[modify] https://crrev.com/01ab158fd9c6ce1f1444f616f06fd4fb409bdb50/scripts/accept_lang.list

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01f386bfb5e4de91a53f06fdae37d62415ee874e

commit 01f386bfb5e4de91a53f06fdae37d62415ee874e
Author: Jungshik Shin <jshin@chromium.org>
Date: Tue Jul 18 23:17:09 2017

Roll ICU to 1fec0c8 and add 4 languages to A-L list.

This will increase the ICU data size by ~21 kB (desktop) and ~18 kB (Android
and iOS).

1fec0c8: Add Spanish, Arabic and English variants for better locale
         coverage in v8's Intl API support.
01ab158: Add the display names of 4 languages for Translate UI.

 http://chromium.googlesource.com/chromium/deps/icu.git/+log/b971435..1fec0c8

BUG= chromium:733398 , chromium:732762 
TEST=See the bug
TBR=littledan@chromium.org,yyushkina@chromium.org

Change-Id: Ib45942a069511b359495e73ad611b99d381a1e8f
Reviewed-on: https://chromium-review.googlesource.com/576383
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487657}
[modify] https://crrev.com/01f386bfb5e4de91a53f06fdae37d62415ee874e/DEPS
[modify] https://crrev.com/01f386bfb5e4de91a53f06fdae37d62415ee874e/chrome/browser/chromeos/locale_change_guard_unittest.cc
[modify] https://crrev.com/01f386bfb5e4de91a53f06fdae37d62415ee874e/ui/base/l10n/l10n_util.cc