My a-bit-hacky implementation for automatic partial loading with angular-translate.

tnbi18n.js

(function () {
        //NB: parts have been stripped to improve readability

	var l10nModule = angular.module( 'tnbl10n', [
		'ngResource',
		'sessionstatus' //This is a module we use here to get the language in use
	] );


	//Backend link
	l10nModule.factory('TranslationWebService', function($resource, sessionStatus) {
		return $resource('api/i18n/translations-' + sessionStatus.lang);
	});

	l10nModule.config( function ( $translateProvider ) {
		$translateProvider.useMissingTranslationHandler( 'myCustomHandlerFactory' );
		$translateProvider.useLoader('customLoader');
		$translateProvider.determinePreferredLanguage();
		//$translateProvider.preferredLanguage('en');
		$translateProvider.usePostCompiling(true);
	} );

	l10nModule.value( 'translations', {
		fr : {
			unresolvedTranslations: {},
			queuedTranslations: {},
			requestedTranslations: {},
			loadedTranslatations: {}
		},
		en : {
			//Keeps missing translations which hasn't been loaded yet
			unresolvedTranslations: {},
			//missing translations for which a server request has been sent and we're waiting for
			queuedTranslations: {},
			//missing translations for which a server request has been sent (from the beginning)
			//this is useful when a translation doesn't exist, so the system doesn't loop requesting for it 
			requestedTranslations: {},
			//translatations successfully loaded
			loadedTranslatations: {}
		}
	} );
	
	l10nModule.factory( 'myCustomHandlerFactory', function ( $translate, customLoader, translations, sessionStatus ) {
		// has to return a function which gets a tranlation ID
		return function ( translationID , lang) {
			if(!lang){
				lang = sessionStatus.lang;
			}
			translations[lang].unresolvedTranslations[translationID] = true;
			//The translation doesn't exist, just print its ID
			if ( translations[lang].requestedTranslations[translationID] ) return translationID;
			if ( translations[lang].queuedTranslations[translationID] ) return;
			//The translation has to be loaded, in the meantime, we show '...'
			//which is a clue for the user that content is being loaded
			customLoader.requireRefresh(translationID);
			return '...';
		};
	} );

	l10nModule.factory('customLoader', function ( $resource, $http, $q, $log, $translate, $locale, _, translations) {
		// return loaderFn
		var customLoader = function (options) {
			var deferred = $q.defer();
			// do something with $http, $q and key to load localization files

			$log.log( 'there customLoader:', options );

			var lang = options.key;
			var url = 'api/i18n/partialtranslations-:locale';
			//We extract groups from all missing translations
			var groups = _.uniq(_.keys( translations[lang].unresolvedTranslations ).map(function(key) {
				return key.split('.')[0];
			})).join();
			// Do not send empty requests, possible infinite loops
			if(!groups.length)  deferred.resolve(options.key)
			else
			$resource(url,{locale:lang}).get(
			{
				groups: groups
			},
			
			function(jsonResponse, status, headers, config) {
				//if we just resolved jsonResponse.data here, previously loaded translations would be lost.
				angular.extend( translations[lang].loadedTranslatations, jsonResponse.data );
				deferred.resolve(translations[lang].loadedTranslatations);
			},
			function(jsonResponse, status, headers, config) {
				//NOTE: angular-translation is buggy if we reject here. He will stop triggering the missing translation handler ever after
				deferred.resolve(options.key);
			}
			);

			translations[lang].queuedTranslations = translations[lang].unresolvedTranslations;
			angular.extend( translations[lang].requestedTranslations, translations[lang].unresolvedTranslations );
			translations[lang].unresolvedTranslations = {};

			return deferred.promise;
		};
		
		var LOADER_DEBOUNCE_DELAY = 100;
		
		var debouncedRequireRefresh = _.debounce( function() {
			$translate.refresh();
		} , LOADER_DEBOUNCE_DELAY);
		customLoader.requireRefresh = function() {
			debouncedRequireRefresh();
		};
		return customLoader;
	});






})();