[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/gtsmodel/status.go

@@ -237,3 +237,14 @@ const (
 	// VisibilityDefault is used when no other setting can be found.
 	VisibilityDefault Visibility = VisibilityUnlocked
 )
+
+// Content models the simple string content
+// of a status along with its ContentMap,
+// which contains content entries keyed by
+// BCP47 language tag.
+//
+// Content and/or ContentMap may be zero/nil.
+type Content struct {
+	Content    string
+	ContentMap map[string]string
+}