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The browser language settings page allows a user to add preferred
languages, and only the added languages are candidates for toggling
on for spellchecking. Several of the accept languages come in both
regionally specific form (Brazilian Portuguese, "pt-BR" e.g.) and
generic form (Portuguese, "pt" e.g.). However, the current logic in the
the SpellcheckService class that maps accept language tags to more
complete BCP47 tags specifying Windows spellcheck dictionaries does
not map the platform dictionary name to the generic name if an exact
match is already found. For example, "pt-BR" gets mapped to its exact
match "pt-BR", but not to "pt". Since there is no Hunspell support for
generic "pt", you cannot enable generic Portuguese for spellchecking.

A more insidious issue is seen with Italian. If you install the
Italian (Italy) Windows language pack, you get dictionary support
for "it-IT" and "it-CH" (Italian as spoken in Italy and Switzerland
respectively). Since there is an exact match with the "it-IT" accept
language tag, generic "it" does not get mapped. But since generic "it"
does have Hunspell support, the hybrid spellcheck fallback logic will
kick in here. A user can enable generic Italian for spellcheck but in
fact Hunspell will be used, even with the Italian Windows language
pack in place.

The fix is to add an entry to the dictionary map for the generic
language subtag if a regionally specific platform spellcheck dictionary
is available, and pass the generic subtag to the platform API when
creating a Windows spellchecker. The platform API is smart enough to
recognize the primary language associated with a language pack, so
passing the "pt" subtag will in fact create the "pt-BR" spellchecker.

Note this fix only works when the Windows platform spellchecking
feature is enabled. If using Hunspell exclusively, you still can't
toggle generic languages unless they can be mapped to an available
Hunspell dictionary

Bug: 1105610
Change-Id: Ib995289a55ed8c8c51bf83e3a1ca4118a07814c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310804Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Bruce Long <brlong@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#791808}