diff --git a/src/ui/js/diff2html-ui-base.ts b/src/ui/js/diff2html-ui-base.ts
index 906b97d..7aaaa5f 100644
--- a/src/ui/js/diff2html-ui-base.ts
+++ b/src/ui/js/diff2html-ui-base.ts
@@ -1,4 +1,4 @@
-import { closeTags, nodeStream, mergeStreams } from './highlight.js-helpers';
+import { closeTags, nodeStream, mergeStreams, getLanguage } from './highlight.js-helpers';
 
 import { html, Diff2HtmlConfig, defaultDiff2HtmlConfig } from '../../diff2html';
 import { DiffFile } from '../../types';
@@ -142,7 +142,7 @@ export class Diff2HtmlUI {
       // HACK: help Typescript know that `this.hljs` is defined since we already checked it
       if (this.hljs === null) return;
       const language = file.getAttribute('data-lang');
-      const hljsLanguage = language ? this.hljs.getLanguage(language) : undefined;
+      const hljsLanguage = language ? getLanguage(language) : 'plaintext';
 
       // Collect all the code lines and execute the highlight on them
       const codeLines = file.querySelectorAll('.d2h-code-line-ctn');
@@ -157,7 +157,7 @@ export class Diff2HtmlUI {
 
         const result: HighlightResult = closeTags(
           this.hljs.highlight(text, {
-            language: hljsLanguage?.name || 'plaintext',
+            language: hljsLanguage,
             ignoreIllegals: true,
           }),
         );
diff --git a/src/ui/js/highlight.js-helpers.ts b/src/ui/js/highlight.js-helpers.ts
index 6512194..3bedb49 100644
--- a/src/ui/js/highlight.js-helpers.ts
+++ b/src/ui/js/highlight.js-helpers.ts
@@ -158,3 +158,498 @@ export function closeTags(res: HighlightResult): HighlightResult {
 
   return res;
 }
+
+// Sourced from https://github.com/highlightjs/highlight.js/blob/main/SUPPORTED_LANGUAGES.md and
+// https://github.com/exercism/v2-website/blob/main/config/initializers/prism.rb#L187-L315
+const languagesToExt: { [_: string]: string } = {
+  '1c': '1c',
+  abnf: 'abnf',
+  accesslog: 'accesslog',
+  as: 'actionscript',
+  adb: 'ada',
+  ada: 'ada',
+  ads: 'ada',
+  angelscript: 'angelscript',
+  // asc: 'angelscript',
+  apache: 'apache',
+  applescript: 'applescript',
+  scpt: 'applescript',
+  arcade: 'arcade',
+  cpp: 'cpp',
+  hpp: 'cpp',
+  arduino: 'arduino',
+  ino: 'arduino',
+  armasm: 'armasm',
+  arm: 'armasm',
+  xml: 'xml',
+  html: 'xml',
+  xhtml: 'xml',
+  rss: 'xml',
+  atom: 'xml',
+  xjb: 'xml',
+  xsd: 'xml',
+  xsl: 'xml',
+  plist: 'xml',
+  svg: 'xml',
+  asciidoc: 'asciidoc',
+  adoc: 'asciidoc',
+  asc: 'asciidoc',
+  aspectj: 'aspectj',
+  ahk: 'autohotkey',
+  ahkl: 'autohotkey',
+  au3: 'autoit',
+  avrasm: 'avrasm',
+  awk: 'awk',
+  axapta: 'axapta',
+  'x++': 'axapta',
+  bash: 'bash',
+  sh: 'bash',
+  zsh: 'bash',
+  b: 'basic',
+  bnf: 'bnf',
+  bf: 'brainfuck',
+  c: 'c',
+  h: 'c',
+  cats: 'c',
+  idc: 'c',
+  cal: 'cal',
+  capnproto: 'capnproto',
+  capnp: 'capnproto',
+  ceylon: 'ceylon',
+  clean: 'clean',
+  clj: 'clojure',
+  boot: 'clojure',
+  cl2: 'clojure',
+  cljc: 'clojure',
+  cljs: 'clojure',
+  'cljs.hl': 'clojure',
+  cljscm: 'clojure',
+  cljx: 'clojure',
+  hic: 'clojure',
+  'clojure-repl': 'clojure-repl',
+  cmake: 'cmake',
+  'cmake.in': 'cmake',
+  coffee: 'coffeescript',
+  _coffee: 'coffeescript',
+  cake: 'coffeescript',
+  cjsx: 'coffeescript',
+  iced: 'coffeescript',
+  cson: 'coffeescript',
+  coq: 'coq',
+  cos: 'cos',
+  cls: 'cos',
+  crmsh: 'crmsh',
+  crm: 'crmsh',
+  pcmk: 'crmsh',
+  cr: 'crystal',
+  cs: 'csharp',
+  csx: 'csharp',
+  csp: 'csp',
+  css: 'css',
+  d: 'd',
+  di: 'd',
+  md: 'markdown',
+  markdown: 'markdown',
+  mdown: 'markdown',
+  mdwn: 'markdown',
+  mkd: 'markdown',
+  mkdn: 'markdown',
+  mkdown: 'markdown',
+  ronn: 'markdown',
+  workbook: 'markdown',
+  dart: 'dart',
+  dpr: 'delphi',
+  dfm: 'delphi',
+  pas: 'delphi',
+  pascal: 'delphi',
+  diff: 'diff',
+  patch: 'diff',
+  django: 'django',
+  jinja: 'django',
+  dns: 'dns',
+  zone: 'dns',
+  bind: 'dns',
+  dockerfile: 'dockerfile',
+  docker: 'dockerfile',
+  dos: 'dos',
+  bat: 'dos',
+  cmd: 'dos',
+  dsconfig: 'dsconfig',
+  dts: 'dts',
+  dust: 'dust',
+  dst: 'dust',
+  ebnf: 'ebnf',
+  ex: 'elixir',
+  exs: 'elixir',
+  elm: 'elm',
+  rb: 'ruby',
+  builder: 'ruby',
+  eye: 'ruby',
+  gemspec: 'ruby',
+  god: 'ruby',
+  jbuilder: 'ruby',
+  mspec: 'ruby',
+  pluginspec: 'ruby',
+  podspec: 'ruby',
+  rabl: 'ruby',
+  rake: 'ruby',
+  rbuild: 'ruby',
+  rbw: 'ruby',
+  rbx: 'ruby',
+  ru: 'ruby',
+  ruby: 'ruby',
+  spec: 'ruby',
+  thor: 'ruby',
+  watchr: 'ruby',
+  erb: 'erb',
+  'erlang-repl': 'erlang-repl',
+  erl: 'erlang',
+  'app.src': 'erlang',
+  escript: 'erlang',
+  hrl: 'erlang',
+  xrl: 'erlang',
+  yrl: 'erlang',
+  excel: 'excel',
+  xls: 'excel',
+  xlsx: 'excel',
+  fix: 'fix',
+  flix: 'flix',
+  f90: 'fortran',
+  f: 'fortran',
+  f03: 'fortran',
+  f08: 'fortran',
+  f77: 'fortran',
+  f95: 'fortran',
+  for: 'fortran',
+  fpp: 'fortran',
+  fs: 'fsharp',
+  fsx: 'fsharp',
+  gams: 'gams',
+  gms: 'gams',
+  gauss: 'gauss',
+  gss: 'gauss',
+  gcode: 'gcode',
+  nc: 'gcode',
+  gherkin: 'gherkin',
+  glsl: 'glsl',
+  fp: 'glsl',
+  frag: 'glsl',
+  frg: 'glsl',
+  fsh: 'glsl',
+  fshader: 'glsl',
+  geo: 'glsl',
+  geom: 'glsl',
+  glslv: 'glsl',
+  gshader: 'glsl',
+  shader: 'glsl',
+  tesc: 'glsl',
+  tese: 'glsl',
+  vert: 'glsl',
+  vrx: 'glsl',
+  vsh: 'glsl',
+  vshader: 'glsl',
+  gml: 'gml',
+  go: 'go',
+  bal: 'go',
+  golo: 'golo',
+  gololang: 'golo',
+  gradle: 'gradle',
+  groovy: 'groovy',
+  grt: 'groovy',
+  gtpl: 'groovy',
+  gvy: 'groovy',
+  haml: 'haml',
+  'haml.deface': 'haml',
+  handlebars: 'handlebars',
+  hbs: 'handlebars',
+  'html.hbs': 'handlebars',
+  'html.handlebars': 'handlebars',
+  hs: 'haskell',
+  hsc: 'haskell',
+  idr: 'haskell',
+  purs: 'haskell',
+  hx: 'haxe',
+  hxsl: 'haxe',
+  hsp: 'hsp',
+  htmlbars: 'htmlbars',
+  http: 'http',
+  https: 'http',
+  hy: 'hy',
+  inform7: 'inform7',
+  i7: 'inform7',
+  ini: 'ini',
+  toml: 'ini',
+  cfg: 'ini',
+  prefs: 'ini',
+  // properties: 'ini',
+  irpf90: 'irpf90',
+  isbl: 'isbl',
+  java: 'java',
+  jsp: 'java',
+  js: 'javascript',
+  jsx: 'javascript',
+  _js: 'javascript',
+  bones: 'javascript',
+  es: 'javascript',
+  es6: 'javascript',
+  gs: 'javascript',
+  jake: 'javascript',
+  jsb: 'javascript',
+  jscad: 'javascript',
+  jsfl: 'javascript',
+  jsm: 'javascript',
+  jss: 'javascript',
+  mjs: 'javascript',
+  njs: 'javascript',
+  pac: 'javascript',
+  sjs: 'javascript',
+  ssjs: 'javascript',
+  xsjs: 'javascript',
+  xsjslib: 'javascript',
+  cfc: 'javascript',
+  'jboss-cli': 'jboss-cli',
+  json: 'json',
+  avsc: 'json',
+  geojson: 'json',
+  gltf: 'json',
+  'JSON-tmLanguage': 'json',
+  jsonl: 'json',
+  tfstate: 'json',
+  'tfstate.backup': 'json',
+  topojson: 'json',
+  webapp: 'json',
+  webmanifest: 'json',
+  jl: 'julia',
+  'julia-repl': 'julia-repl',
+  kt: 'kotlin',
+  ktm: 'kotlin',
+  kts: 'kotlin',
+  lasso: 'lasso',
+  // ls: 'lasso',
+  lassoscript: 'lasso',
+  tex: 'latex',
+  ldif: 'ldif',
+  leaf: 'leaf',
+  less: 'less',
+  lisp: 'lisp',
+  factor: 'lisp',
+  livecodeserver: 'livecodeserver',
+  ls: 'livescript',
+  _ls: 'livescript',
+  llvm: 'llvm',
+  lsl: 'lsl',
+  lua: 'lua',
+  nse: 'lua',
+  p8: 'lua',
+  pd_lua: 'lua',
+  rbxs: 'lua',
+  wlua: 'lua',
+  mak: 'makefile',
+  make: 'makefile',
+  mk: 'makefile',
+  mkfile: 'makefile',
+  mathematica: 'mathematica',
+  mma: 'mathematica',
+  wl: 'mathematica',
+  matlab: 'matlab',
+  maxima: 'maxima',
+  mel: 'mel',
+  mercury: 'mercury',
+  mipsasm: 'mipsasm',
+  miz: 'mizar',
+  voc: 'mizar',
+  al: 'perl',
+  cgi: 'perl',
+  fcgi: 'perl',
+  perl: 'perl',
+  ph: 'perl',
+  plx: 'perl',
+  pl: 'perl',
+  pm: 'perl',
+  psgi: 'perl',
+  t: 'perl',
+  mojolicious: 'mojolicious',
+  monkey: 'monkey',
+  monkey2: 'monkey',
+  moonscript: 'moonscript',
+  moon: 'moonscript',
+  n1ql: 'n1ql',
+  nginxconf: 'nginx',
+  nim: 'nim',
+  nimrod: 'nim',
+  nix: 'nix',
+  nsi: 'nsis',
+  nsh: 'nsis',
+  m: 'objectivec',
+  objc: 'objectivec',
+  mm: 'objectivec',
+  'obj-c': 'objectivec',
+  'obj-c++': 'objectivec',
+  'objective-c++': 'objectivec',
+  fun: 'ocaml',
+  sig: 'ocaml',
+  // sml: 'ocaml',
+  ml: 'ocaml',
+  mli: 'ocaml',
+  eliom: 'ocaml',
+  eliomi: 'ocaml',
+  ml4: 'ocaml',
+  mll: 'ocaml',
+  mly: 'ocaml',
+  openscad: 'openscad',
+  oxygene: 'oxygene',
+  parser3: 'parser3',
+  pf: 'pf',
+  'pf.conf': 'pf',
+  pgsql: 'pgsql',
+  postgres: 'pgsql',
+  postgresql: 'pgsql',
+  php: 'php',
+  aw: 'php',
+  ctp: 'php',
+  inc: 'php',
+  php3: 'php',
+  php4: 'php',
+  php5: 'php',
+  phps: 'php',
+  phpt: 'php',
+  'php-template': 'php-template',
+  plaintext: 'plaintext',
+  txt: 'plaintext',
+  text: 'plaintext',
+  pony: 'pony',
+  ps: 'powershell',
+  ps1: 'powershell',
+  psd1: 'powershell',
+  psm1: 'powershell',
+  pde: 'processing',
+  profile: 'profile',
+  pro: 'prolog',
+  prolog: 'prolog',
+  yap: 'prolog',
+  properties: 'properties',
+  proto: 'protobuf',
+  puppet: 'puppet',
+  pp: 'puppet',
+  purebasic: 'purebasic',
+  py: 'python',
+  bzl: 'python',
+  gyp: 'python',
+  gypi: 'python',
+  lmi: 'python',
+  py3: 'python',
+  pyde: 'python',
+  pyi: 'python',
+  pyp: 'python',
+  pyt: 'python',
+  pyw: 'python',
+  rpy: 'python',
+  tac: 'python',
+  wsgi: 'python',
+  xpy: 'python',
+  'python-repl': 'python-repl',
+  pycon: 'python-repl',
+  q: 'q',
+  k: 'q',
+  kdb: 'q',
+  qml: 'qml',
+  r: 'r',
+  rd: 'r',
+  rsx: 'r',
+  reasonml: 'reasonml',
+  re: 'reasonml',
+  rib: 'rib',
+  roboconf: 'roboconf',
+  graph: 'roboconf',
+  instances: 'roboconf',
+  routeros: 'routeros',
+  rsl: 'rsl',
+  ruleslanguage: 'ruleslanguage',
+  rs: 'rust',
+  'rs.in': 'rust',
+  sas: 'sas',
+  // pony: 'scala',
+  scala: 'scala',
+  kojo: 'scala',
+  sbt: 'scala',
+  sc: 'scala',
+  scm: 'scheme',
+  sch: 'scheme',
+  sld: 'scheme',
+  sls: 'scheme',
+  sps: 'scheme',
+  ss: 'scheme',
+  rkt: 'scheme',
+  scilab: 'scilab',
+  scss: 'scss',
+  shell: 'shell',
+  smali: 'smali',
+  st: 'smalltalk',
+  sml: 'sml',
+  sqf: 'sqf',
+  sql: 'sql',
+  cql: 'sql',
+  ddl: 'sql',
+  mysql: 'sql',
+  prc: 'sql',
+  tab: 'sql',
+  udf: 'sql',
+  viw: 'sql',
+  stan: 'stan',
+  stanfuncs: 'stan',
+  stata: 'stata',
+  step21: 'step21',
+  step: 'step21',
+  stp: 'step21',
+  styl: 'stylus',
+  subunit: 'subunit',
+  swift: 'swift',
+  taggerscript: 'taggerscript',
+  yml: 'yaml',
+  mir: 'yaml',
+  reek: 'yaml',
+  rviz: 'yaml',
+  'sublime-syntax': 'yaml',
+  syntax: 'yaml',
+  yaml: 'yaml',
+  'yaml-tmlanguage': 'yaml',
+  'yml.mysql': 'yaml',
+  tap: 'tap',
+  tcl: 'tcl',
+  adp: 'tcl',
+  tm: 'tcl',
+  thrift: 'thrift',
+  tp: 'tp',
+  twig: 'twig',
+  craftcms: 'twig',
+  ts: 'typescript',
+  tsx: 'typescript',
+  vala: 'vala',
+  vbnet: 'vbnet',
+  vb: 'vbnet',
+  vbscript: 'vbscript',
+  vbs: 'vbscript',
+  'vbscript-html': 'vbscript-html',
+  v: 'verilog',
+  veo: 'verilog',
+  vhdl: 'vhdl',
+  vhd: 'vhdl',
+  vhf: 'vhdl',
+  vhi: 'vhdl',
+  vho: 'vhdl',
+  vhs: 'vhdl',
+  vht: 'vhdl',
+  vhw: 'vhdl',
+  vim: 'vim',
+  x86asm: 'x86asm',
+  xl: 'xl',
+  xquery: 'xquery',
+  xpath: 'xquery',
+  xq: 'xquery',
+  zephir: 'zephir',
+  zep: 'zephir',
+};
+
+export function getLanguage(fileExtension: string): string {
+  return languagesToExt[fileExtension] ?? 'plaintext';
+}