refactor(locale): delay parsing of translations until they're used

While doing some profiling for #2900, I noticed that
`miniflux.app/v2/internal/locale.LoadCatalogMessages` is responsible for more
than 10% of the consumed memory. As most miniflux instances won't have enough
diverse users to use all the available translations at the same time, it
makes sense to load them on demand.

The overhead is a single function call and a check in a map, per call to
translation-related functions.

internal/locale/locale.go

@@ -3,26 +3,24 @@
 
 package locale // import "miniflux.app/v2/internal/locale"
 
-// AvailableLanguages returns the list of available languages.
-func AvailableLanguages() map[string]string {
-	return map[string]string{
-		"en_US": "English",
-		"es_ES": "Español",
-		"fr_FR": "Français",
-		"de_DE": "Deutsch",
-		"pl_PL": "Polski",
-		"pt_BR": "Português Brasileiro",
-		"zh_CN": "简体中文",
-		"zh_TW": "繁體中文",
-		"nl_NL": "Nederlands",
-		"ru_RU": "Русский",
-		"it_IT": "Italiano",
-		"ja_JP": "日本語",
-		"tr_TR": "Türkçe",
-		"el_EL": "Ελληνικά",
-		"fi_FI": "Suomi",
-		"hi_IN": "हिन्दी",
-		"uk_UA": "Українська",
-		"id_ID": "Bahasa Indonesia",
-	}
+// AvailableLanguages is the list of available languages.
+var AvailableLanguages = map[string]string{
+	"en_US": "English",
+	"es_ES": "Español",
+	"fr_FR": "Français",
+	"de_DE": "Deutsch",
+	"pl_PL": "Polski",
+	"pt_BR": "Português Brasileiro",
+	"zh_CN": "简体中文",
+	"zh_TW": "繁體中文",
+	"nl_NL": "Nederlands",
+	"ru_RU": "Русский",
+	"it_IT": "Italiano",
+	"ja_JP": "日本語",
+	"tr_TR": "Türkçe",
+	"el_EL": "Ελληνικά",
+	"fi_FI": "Suomi",
+	"hi_IN": "हिन्दी",
+	"uk_UA": "Українська",
+	"id_ID": "Bahasa Indonesia",
 }