diff --git a/gulpfile.ts b/gulpfile.ts
index fa1155878c..49a80879d2 100644
--- a/gulpfile.ts
+++ b/gulpfile.ts
@@ -8,12 +8,12 @@ import * as gutil from 'gulp-util';
 import * as ts from 'gulp-typescript';
 const sourcemaps = require('gulp-sourcemaps');
 import tslint from 'gulp-tslint';
-import cssnano = require('gulp-cssnano');
+const cssnano = require('gulp-cssnano');
 import * as uglifyComposer from 'gulp-uglify/composer';
 import pug = require('gulp-pug');
 import * as rimraf from 'rimraf';
 import chalk from 'chalk';
-import imagemin = require('gulp-imagemin');
+const imagemin = require('gulp-imagemin');
 import * as rename from 'gulp-rename';
 import * as mocha from 'gulp-mocha';
 import * as replace from 'gulp-replace';
diff --git a/locales/index.ts b/locales/index.ts
index 319d178e0a..3b4f76b06a 100644
--- a/locales/index.ts
+++ b/locales/index.ts
@@ -5,12 +5,16 @@
 import * as fs from 'fs';
 import * as yaml from 'js-yaml';
 
-const loadLang = lang => yaml.safeLoad(
-	fs.readFileSync(`./locales/${lang}.yml`, 'utf-8'));
+export type LangKey = 'de' | 'en' | 'fr' | 'ja' | 'pl';
+export type LocaleObjectChildren = LocaleObject | string | undefined;
+export type LocaleObject = {[key: string]: LocaleObjectChildren };
+
+const loadLang = (lang: LangKey) => yaml.safeLoad(
+	fs.readFileSync(`./locales/${lang}.yml`, 'utf-8')) as LocaleObject;
 
 const native = loadLang('ja');
 
-const langs = {
+const langs: {[key in LangKey]: LocaleObject} = {
 	'de': loadLang('de'),
 	'en': loadLang('en'),
 	'fr': loadLang('fr'),
@@ -23,4 +27,8 @@ Object.entries(langs).map(([, locale]) => {
 	locale = Object.assign({}, native, locale);
 });
 
+export function isAvailableLanguage(lang: string): lang is LangKey {
+	return lang in langs;
+}
+
 export default langs;
diff --git a/src/build/fa.ts b/src/build/fa.ts
index 111c19ae66..077bb51e6d 100644
--- a/src/build/fa.ts
+++ b/src/build/fa.ts
@@ -3,18 +3,18 @@
  */
 
 import * as fontawesome from '@fortawesome/fontawesome';
-import * as regular from '@fortawesome/fontawesome-free-regular';
-import * as solid from '@fortawesome/fontawesome-free-solid';
-import * as brands from '@fortawesome/fontawesome-free-brands';
+import regular from '@fortawesome/fontawesome-free-regular';
+import solid from '@fortawesome/fontawesome-free-solid';
+import brands from '@fortawesome/fontawesome-free-brands';
 
 fontawesome.library.add(regular, solid, brands);
 
 export const pattern = /%fa:(.+?)%/g;
 
-export const replacement = (match, key) => {
+export const replacement = (match: string, key: string) => {
 	const args = key.split(' ');
 	let prefix = 'fas';
-	const classes = [];
+	const classes: string[] = [];
 	let transform = '';
 	let name;
 
@@ -34,12 +34,12 @@ export const replacement = (match, key) => {
 		}
 	});
 
-	const icon = fontawesome.icon({ prefix, iconName: name }, {
-		classes: classes
+	const icon = fontawesome.icon({ prefix, iconName: name } as fontawesome.IconLookup, {
+		classes: classes,
+		transform: fontawesome.parse.transform(transform)
 	});
 
 	if (icon) {
-		icon.transform = fontawesome.parse.transform(transform);
 		return `<i data-fa class="${name}">${icon.html[0]}</i>`;
 	} else {
 		console.warn(`'${name}' not found in fa`);
diff --git a/src/build/i18n.ts b/src/build/i18n.ts
index 35854055d0..308ff0da00 100644
--- a/src/build/i18n.ts
+++ b/src/build/i18n.ts
@@ -2,7 +2,7 @@
  * Replace i18n texts
  */
 
-import locale from '../../locales';
+import locale, { isAvailableLanguage, LocaleObject, LocaleObjectChildren } from '../../locales';
 
 export default class Replacer {
 	private lang: string;
@@ -16,19 +16,19 @@ export default class Replacer {
 		this.replacement = this.replacement.bind(this);
 	}
 
-	private get(path: string, key: string) {
-		const texts = locale[this.lang];
-
-		if (texts == null) {
+	private get(path: string, key: string): string {
+		if (!isAvailableLanguage(this.lang)) {
 			console.warn(`lang '${this.lang}' is not supported`);
 			return key; // Fallback
 		}
 
-		let text = texts;
+		const texts = locale[this.lang];
+
+		let text: LocaleObjectChildren = texts;
 
 		if (path) {
 			if (text.hasOwnProperty(path)) {
-				text = text[path];
+				text = text[path] as LocaleObject;
 			} else {
 				console.warn(`path '${path}' not found in '${this.lang}'`);
 				return key; // Fallback
@@ -38,7 +38,7 @@ export default class Replacer {
 		// Check the key existance
 		const error = key.split('.').some(k => {
 			if (text.hasOwnProperty(k)) {
-				text = text[k];
+				text = (text as LocaleObject)[k];
 				return false;
 			} else {
 				return true;
@@ -48,12 +48,15 @@ export default class Replacer {
 		if (error) {
 			console.warn(`key '${key}' not found in '${path}' of '${this.lang}'`);
 			return key; // Fallback
+		} else if (typeof text !== "string") {
+			console.warn(`key '${key}' is not string in '${path}' of '${this.lang}'`);
+			return key; // Fallback
 		} else {
 			return text;
 		}
 	}
 
-	public replacement(match, key) {
+	public replacement(match: string, key: string) {
 		let path = null;
 
 		if (key.indexOf('|') != -1) {
diff --git a/src/client/docs/api/gulpfile.ts b/src/client/docs/api/gulpfile.ts
index 9980ede231..0eb8b88287 100644
--- a/src/client/docs/api/gulpfile.ts
+++ b/src/client/docs/api/gulpfile.ts
@@ -19,9 +19,10 @@ import generateVars from '../vars';
 
 const langs = Object.keys(locales);
 
-const kebab = string => string.replace(/([a-z])([A-Z])/g, '$1-$2').replace(/\s+/g, '-').toLowerCase();
+const kebab = (string: string) => string.replace(/([a-z])([A-Z])/g, '$1-$2').replace(/\s+/g, '-').toLowerCase();
 
-const parseParam = param => {
+// WIP type
+const parseParam = (param: any) => {
 	const id = param.type.match(/^id\((.+?)\)|^id/);
 	const entity = param.type.match(/^entity\((.+?)\)/);
 	const isObject = /^object/.test(param.type);
@@ -57,7 +58,7 @@ const parseParam = param => {
 	return param;
 };
 
-const sortParams = params => {
+const sortParams = (params: Array<{name: string}>) => {
 	params.sort((a, b) => {
 		if (a.name < b.name)
 			return -1;
@@ -68,14 +69,15 @@ const sortParams = params => {
 	return params;
 };
 
-const extractDefs = params => {
-	let defs = [];
+// WIP type
+const extractDefs = (params: any[]) => {
+	let defs: any[] = [];
 
 	params.forEach(param => {
 		if (param.def) {
 			defs.push({
 				name: param.defName,
-				params: sortParams(param.def.map(p => parseParam(p)))
+				params: sortParams(param.def.map((p: any) => parseParam(p)))
 			});
 
 			const childDefs = extractDefs(param.def);
@@ -109,8 +111,10 @@ gulp.task('doc:api:endpoints', async () => {
 					path: ep.endpoint
 				},
 				desc: ep.desc,
+				// @ts-ignore
 				params: sortParams(ep.params.map(p => parseParam(p))),
 				paramDefs: extractDefs(ep.params),
+				// @ts-ignore
 				res: ep.res ? sortParams(ep.res.map(p => parseParam(p))) : null,
 				resDefs: ep.res ? extractDefs(ep.res) : null,
 			};
@@ -155,7 +159,8 @@ gulp.task('doc:api:entities', async () => {
 			const vars = {
 				name: entity.name,
 				desc: entity.desc,
-				props: sortParams(entity.props.map(p => parseParam(p))),
+				// WIP type
+				props: sortParams(entity.props.map((p: any) => parseParam(p))),
 				propDefs: extractDefs(entity.props),
 			};
 			langs.forEach(lang => {
diff --git a/src/client/docs/gulpfile.ts b/src/client/docs/gulpfile.ts
index 56bf6188c8..4683a04659 100644
--- a/src/client/docs/gulpfile.ts
+++ b/src/client/docs/gulpfile.ts
@@ -8,8 +8,8 @@ import * as glob from 'glob';
 import * as gulp from 'gulp';
 import * as pug from 'pug';
 import * as mkdirp from 'mkdirp';
-import stylus = require('gulp-stylus');
-import cssnano = require('gulp-cssnano');
+const stylus = require('gulp-stylus');
+const cssnano = require('gulp-cssnano');
 
 import I18nReplacer from '../../build/i18n';
 import fa from '../../build/fa';
diff --git a/src/client/docs/vars.ts b/src/client/docs/vars.ts
index 32b961aaa9..93082767e3 100644
--- a/src/client/docs/vars.ts
+++ b/src/client/docs/vars.ts
@@ -38,7 +38,7 @@ export default async function(): Promise<{ [key: string]: any }> {
 		vars['docs'][name]['title'][lang] = fs.readFileSync(x, 'utf-8').match(/^h1 (.+?)\r?\n/)[1];
 	});
 
-	vars['kebab'] = string => string.replace(/([a-z])([A-Z])/g, '$1-$2').replace(/\s+/g, '-').toLowerCase();
+	vars['kebab'] = (string: string) => string.replace(/([a-z])([A-Z])/g, '$1-$2').replace(/\s+/g, '-').toLowerCase();
 
 	vars['config'] = config;
 
diff --git a/yarn.lock b/yarn.lock
index a99e498628..8b12a99c95 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2,21 +2,33 @@
 # yarn lockfile v1
 
 
-"@fortawesome/fontawesome-free-brands@5.0.2":
-  version "5.0.2"
-  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-brands/-/fontawesome-free-brands-5.0.2.tgz#a1cc602eec40a379a3dd8a44c78b31110dd3d3d3"
+"@fortawesome/fontawesome-common-types@^0.1.7":
+  version "0.1.7"
+  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-common-types/-/fontawesome-common-types-0.1.7.tgz#4336c4b06d0b5608ff1215464b66fcf9f4795284"
 
-"@fortawesome/fontawesome-free-regular@5.0.2":
-  version "5.0.2"
-  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-regular/-/fontawesome-free-regular-5.0.2.tgz#429af86bed14689f87648e6322983c65c782c017"
+"@fortawesome/fontawesome-free-brands@5.0.13":
+  version "5.0.13"
+  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-brands/-/fontawesome-free-brands-5.0.13.tgz#4d15ff4e1e862d5e4a4df3654f8e8acbd47e9c09"
+  dependencies:
+    "@fortawesome/fontawesome-common-types" "^0.1.7"
 
-"@fortawesome/fontawesome-free-solid@5.0.2":
-  version "5.0.2"
-  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-solid/-/fontawesome-free-solid-5.0.2.tgz#090ce2c59dd5ec76983f3da8a43e1ab0321b42d5"
+"@fortawesome/fontawesome-free-regular@5.0.13":
+  version "5.0.13"
+  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-regular/-/fontawesome-free-regular-5.0.13.tgz#eb78c30184e3f456a423a1dcfa0f682f7b50de4a"
+  dependencies:
+    "@fortawesome/fontawesome-common-types" "^0.1.7"
 
-"@fortawesome/fontawesome@1.0.1":
-  version "1.0.1"
-  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome/-/fontawesome-1.0.1.tgz#8ac60e1e7b437889baf9c9d6e3a61ef3b637170d"
+"@fortawesome/fontawesome-free-solid@5.0.13":
+  version "5.0.13"
+  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome-free-solid/-/fontawesome-free-solid-5.0.13.tgz#24b61aaf471a9d34a5364b052d64a516285ba894"
+  dependencies:
+    "@fortawesome/fontawesome-common-types" "^0.1.7"
+
+"@fortawesome/fontawesome@1.1.8":
+  version "1.1.8"
+  resolved "https://registry.yarnpkg.com/@fortawesome/fontawesome/-/fontawesome-1.1.8.tgz#75fe66a60f95508160bb16bd781ad7d89b280f5b"
+  dependencies:
+    "@fortawesome/fontawesome-common-types" "^0.1.7"
 
 "@gulp-sourcemaps/identity-map@1.X":
   version "1.0.1"