diff --git a/I18n/EnvExtension.lean b/I18n/EnvExtension.lean
index 14a1ad4..5ba6c99 100644
--- a/I18n/EnvExtension.lean
+++ b/I18n/EnvExtension.lean
@@ -136,15 +136,15 @@ def readLanguageConfig (lang? : Option Language := none) : IO LanguageState := d
 This extension holds the loaded translations `sourceLang` to `lang`.
 It is up to the developer to keep it in sync with the `languageExt`.
  -/
-initialize translationExt : SimplePersistentEnvExtension (String × String) (HashMap String String)
+initialize translationExt : SimplePersistentEnvExtension (String × String) (Std.HashMap String String)
   ← registerSimplePersistentEnvExtension {
       name := `i18n_translations
       addEntryFn := fun hm (x : String × String) => hm.insert x.1 x.2
-      addImportedFn := fun arr => HashMap.ofList (arr.concatMap id).toList }
+      addImportedFn := fun arr => Std.HashMap.ofList (arr.concatMap id).toList }
 
 /--
 Get the translations from the environment. It is a `HashMap String String`
 mapping from the untranslated string to the translation.
 -/
-def getTranslations [Monad m] [MonadEnv m] : m (HashMap String String) := do
+def getTranslations [Monad m] [MonadEnv m] : m (Std.HashMap String String) := do
   return translationExt.getState (← getEnv)
diff --git a/I18n/PO/Read.lean b/I18n/PO/Read.lean
index 9952852..7307b38 100644
--- a/I18n/PO/Read.lean
+++ b/I18n/PO/Read.lean
@@ -2,11 +2,14 @@ import I18n.PO.Definition
 
 /-! # Parse PO-Files
 
-The main function of this file is `POFile.read`, which uses `Parsec` to
+The main function of this file is `POFile.read`,
+which uses `Std.Internal.Parsec.String.Parser` to
 parse a PO-file and returns a `POFile`.
 -/
 
 open Lean
+open Std.Internal.Parsec
+open Std.Internal.Parsec.String
 
 namespace I18n
 
@@ -24,8 +27,6 @@ def parseRefs (ref : String) : List (String × Option Nat) :=
 def parseFlags (flags : String) : List String :=
   flags.split (· = ',') |>.map (·.trim)
 
-open Parsec
-
 /-- Internal function used by `ws_maxOneLF` -/
 partial def skipWs_maxOneLF (it : String.Iterator) (foundLF := false) : String.Iterator :=
   if it.hasNext then
@@ -44,12 +45,12 @@ partial def skipWs_maxOneLF (it : String.Iterator) (foundLF := false) : String.I
 
 /-- Consume zero or more whitespace characters, but maximal one newline character -/
 @[inline]
-def ws_maxOneLF : Parsec Unit := fun it =>
+def ws_maxOneLF : Parser Unit := fun it =>
   .success (skipWs_maxOneLF it) ()
 
 /-- Parse the next character as an escaped code. -/
-def escapedChar : Parsec Char := do
-  let c ← anyChar
+def escapedChar : Parser Char := do
+  let c ← any
   match c with
   | '\\' => return '\\'
   | '"'  => return '"'
@@ -68,7 +69,7 @@ def escapedChar : Parsec Char := do
 Parses the content of a PO-file string, not including the initial `"`.
 Accepts multiline strings which are closed and reopened around each line-break
  -/
-partial def strCore (acc : String := "") : Parsec String := do
+partial def strCore (acc : String := "") : Parser String := do
   let c ← peek!
   if c = '"' then
     -- end of string found. try to look for continuation on next line
@@ -88,7 +89,7 @@ partial def strCore (acc : String := "") : Parsec String := do
       return acc
   else
     -- process character inside the string
-    let c ← anyChar
+    let c ← any
     if c = '\\' then
       strCore (acc.push (← escapedChar))
     else
@@ -96,14 +97,14 @@ partial def strCore (acc : String := "") : Parsec String := do
 
 /-- Helper function to peek at the second character coming up. -/
 @[inline]
-def peek2? : Parsec (Option Char) := fun it =>
+def peek2? : Parser (Option Char) := fun it =>
   if it.next.hasNext then
     .success it it.next.curr
   else
     .success it none
 
 @[inherit_doc peek2?, inline]
-def peek2! : Parsec Char := do
+def peek2! : Parser Char := do
   let some c ← peek2? | fail unexpectedEndOfInput
   return c
 
@@ -111,7 +112,7 @@ def peek2! : Parsec Char := do
 `f` is a character that should directly follow the '#'
 
 TODO: Implement whitespace `f` instead of just `f = ' '`. -/
-partial def commentCore (acc : String := "") (f : Char) : Parsec String := do
+partial def commentCore (acc : String := "") (f : Char) : Parser String := do
   let c ← peek!
   if c = '\n' then
     -- A comment can continue on the next line if that line starts with `#_` where `_`
@@ -136,16 +137,16 @@ partial def commentCore (acc : String := "") (f : Char) : Parsec String := do
       -- next line does not contain a comment
       return acc
   else
-    let c ← anyChar
+    let c ← any
     if c = '\\' then
       commentCore (acc.push (← escapedChar)) f
     else
       commentCore (acc.push c) f
 
 /-- Returns the content of a comment. -/
-partial def commentAux (acc : POEntry) : Parsec POEntry := do
+partial def commentAux (acc : POEntry) : Parser POEntry := do
   -- This first character specifies the sort of comment, see po specification.
-  let f ← anyChar
+  let f ← any
   if f = '\t' ∨ f = '\n' ∨ f = '\r' ∨ f = ' ' then
     -- whitespace
     ws
@@ -201,7 +202,7 @@ partial def commentAux (acc : POEntry) : Parsec POEntry := do
 /-- Parse an PO-entry.
 
 Note: This assumes that there are no leading spaces on any lines. Is that fine? -/
-partial def parseEntry (acc : Option POEntry := none) : Parsec POEntry := do
+partial def parseEntry (acc : Option POEntry := none) : Parser POEntry := do
   let acc : POEntry := match acc with
   | some acc => acc
   | none => { msgId := "" }
@@ -240,7 +241,7 @@ partial def parseEntry (acc : Option POEntry := none) : Parsec POEntry := do
 
 /-- Core for `I18n.POFile.Parser.parseFile` -/
 partial def parseFileCore (entries : Array POEntry := #[]) (header : Option POHeaderEntry := none) :
-    Parsec POFile := do
+    Parser POFile := do
   ws
   match (← peek?) with
   | some _ =>
@@ -265,8 +266,8 @@ partial def parseFileCore (entries : Array POEntry := #[]) (header : Option POHe
         language          := "" }
     return {entries := entries, header := header}
 
-/-- Parsec parser for PO files. -/
-def parseFile : Parsec POFile := do
+/-- Parser for PO files. -/
+def parseFile : Parser POFile := do
   ws
   let res ← parseFileCore
   eof
@@ -277,8 +278,8 @@ end Parser
 /-- Parse the content of a PO file. -/
 def parse (s : String) : Except String POFile :=
   match POFile.Parser.parseFile s.mkIterator with
-  | Parsec.ParseResult.success _ res => Except.ok res
-  | Parsec.ParseResult.error it err  => Except.error s!"offset {repr it.i.byteIdx}: {err}"
+  | .success _ res => Except.ok res
+  | .error it err  => Except.error s!"offset {repr it.i.byteIdx}: {err}"
 
 end POFile
 
diff --git a/I18n/PO/Write.lean b/I18n/PO/Write.lean
index 2024ede..e6e1985 100644
--- a/I18n/PO/Write.lean
+++ b/I18n/PO/Write.lean
@@ -63,6 +63,9 @@ def toString (e : POEntry) : String := Id.run do
 
 instance : ToString POEntry := ⟨POEntry.toString⟩
 
+-- TODO: Parse the header entry!
+set_option linter.unusedVariables false in
+
 /-- Paring the header entry into a `POHeaderEntry`. -/
 def toPOHeaderEntry (header : POEntry): POHeaderEntry := Id.run do
   return {
diff --git a/I18n/Template.lean b/I18n/Template.lean
index 677a17a..dca2315 100644
--- a/I18n/Template.lean
+++ b/I18n/Template.lean
@@ -59,7 +59,7 @@ def createTemplate : CommandElabM Unit := do
   let keys := untranslatedKeysExt.getState (← getEnv)
 
   -- there might be multiple keys with identical msgId, which we need to merge
-  let groupedEntries : HashMap String (Array POEntry) := keys.groupByKey (·.msgId)
+  let groupedEntries : Std.HashMap String (Array POEntry) := keys.groupByKey (·.msgId)
   let mergedKeys : Array POEntry := groupedEntries.toArray.map (fun (_msgId, entries) =>
     POEntry.mergeMetaDataList entries.toList)
 
diff --git a/I18n/Translate.lean b/I18n/Translate.lean
index dd80b90..d3bf566 100644
--- a/I18n/Translate.lean
+++ b/I18n/Translate.lean
@@ -74,7 +74,7 @@ def _root_.String.translate [Monad m] [MonadEnv m] [MonadLog m] [AddMessageConte
   let sTranslated ← if langConfig.lang == langConfig.sourceLang then
     pure s
   else
-    match (← getTranslations).find? s with
+    match (← getTranslations)[s]? with
     | none =>
       -- Print a warning that the translation has not been found
       logWarning s!"No translation ({langConfig.lang}) found for: {s}"
diff --git a/lake-manifest.json b/lake-manifest.json
index 3bc5488..41a3d4b 100644
--- a/lake-manifest.json
+++ b/lake-manifest.json
@@ -15,10 +15,10 @@
    "type": "git",
    "subDir": null,
    "scope": "",
-   "rev": "9c6c2d647e57b2b7a0b42dd8080c698bd33a1b6f",
+   "rev": "fa571ea02a804b52a59e58012b1e21fe4f0514f2",
    "name": "batteries",
    "manifestFile": "lake-manifest.json",
-   "inputRev": "v4.11.0",
+   "inputRev": "v4.12.0-rc1",
    "inherited": false,
    "configFile": "lakefile.lean"}],
  "name": "i18n",
diff --git a/lean-toolchain b/lean-toolchain
index 5a9c76d..98556ba 100644
--- a/lean-toolchain
+++ b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:v4.11.0
+leanprover/lean4:v4.12.0-rc1