[feature] Federate status language in and out (#2366)

* [feature] Federate status language in + out

* go fmt

* tests, little fix

* improve comments

* unnest a bit

* avoid unnecessary nil check

* use more descriptive variable for contentMap

* prefer instance languages when selecting from contentMap

* update docs to reflect lang selection

* rename rdfLangString -> rdfLangs

* update comments to mention Pollable

* iter through slice instead of map

internal/typeutils/astointernal.go

@@ -244,9 +244,15 @@ func (c *Converter) ASStatusToStatus(ctx context.Context, statusable ap.Statusab
 	}
 
 	// status.Content
+	// status.Language
 	//
-	// The (html-formatted) content of this status.
-	status.Content = ap.ExtractContent(statusable)
+	// Many implementations set both content
+	// and contentMap; we can use these to
+	// infer the language of the status.
+	status.Content, status.Language = ContentToContentLanguage(
+		ctx,
+		ap.ExtractContent(statusable),
+	)
 
 	// status.Attachments
 	//
@@ -396,9 +402,6 @@ func (c *Converter) ASStatusToStatus(ctx context.Context, statusable ap.Statusab
 		return &s
 	}()
 
-	// language
-	// TODO: we might be able to extract this from the contentMap field
-
 	// ActivityStreamsType
 	status.ActivityStreamsType = statusable.GetTypeName()
 
@@ -707,7 +710,7 @@ func (c *Converter) ASFlagToReport(ctx context.Context, flaggable ap.Flaggable)
 	// For Mastodon, this will just be a string, or nothing.
 	// In Misskey's case, it may also contain the URLs of
 	// one or more reported statuses, so extract these too.
-	content := ap.ExtractContent(flaggable)
+	content := ap.ExtractContent(flaggable).Content
 	statusURIs := []*url.URL{}
 	inlineURLs := misskeyReportInlineURLs(content)
 	statusURIs = append(statusURIs, inlineURLs...)