[feature] Set/show instance language(s); show post language on frontend (#2362)

* update go text, include text/display

* [feature] Set instance langs, show post lang on frontend

* go fmt

* WebGet

* set language for whole article, don't use FA icon

* mention instance languages + other optional config vars

* little tweak

* put languages in config properly

* warn log language parse

* change some naming around

* tidy up validate a bit

* lint

* rename LanguageTmpl in template

test/envparsing.sh

@@ -87,6 +87,10 @@ EXPECT=$(cat << "EOF"
     "instance-expose-suspended-web": true,
     "instance-federation-mode": "allowlist",
     "instance-inject-mastodon-version": true,
+    "instance-languages": [
+        "nl",
+        "en-GB"
+    ],
     "landing-page-user": "admin",
     "letsencrypt-cert-dir": "/gotosocial/storage/certs",
     "letsencrypt-email-address": "",
@@ -202,6 +206,7 @@ GTS_INSTANCE_EXPOSE_PUBLIC_TIMELINE=true \
 GTS_INSTANCE_FEDERATION_MODE='allowlist' \
 GTS_INSTANCE_DELIVER_TO_SHARED_INBOXES=false \
 GTS_INSTANCE_INJECT_MASTODON_VERSION=true \
+GTS_INSTANCE_LANGUAGES="nl,en-gb" \
 GTS_ACCOUNTS_ALLOW_CUSTOM_CSS=true \
 GTS_ACCOUNTS_CUSTOM_CSS_LENGTH=5000 \
 GTS_ACCOUNTS_REGISTRATION_OPEN=true \