“Needs language provider” and a missing fabric-language-kotlin: what a missing language provider means and how to fix it

What a language provider is

Forge, NeoForge and Fabric can start mods written in Java by themselves. Mods written in another JVM language, most often Kotlin, usually also need a language provider: a separate library mod that lets the loader start mods written in that language. A mod states which provider it needs and which versions it accepts, and the loader refuses to load it when that provider is absent or too old.

On Forge and NeoForge a mod names its provider in the modLoader line of its mods.toml (neoforge.mods.toml on recent NeoForge versions) and the versions it accepts in loaderVersion. Ordinary Java mods name javafml, the loader's own provider. Kotlin mods built on Kotlin for Forge name kotlinforforge, which that mod supplies.

On Fabric a Kotlin mod built on Fabric Language Kotlin lists fabric-language-kotlin as a dependency in its fabric.mod.json and starts through that mod's kotlin adapter. Fabric checks dependencies before anything starts, so a missing Fabric Language Kotlin shows up as an ordinary missing dependency.

Exact error messages

Lines to match against your server log. Forge and NeoForge print the first two shapes; Fabric prints the third.

How to fix it on Forge or NeoForge

Read the provider id and the version after the colon. kotlinforforge means Kotlin for Forge, and the number is the lowest Kotlin for Forge version the mod accepts. Forge prints the same line whether Kotlin for Forge is missing or too old; the We have found line under it shows what it found. Forge's log also has a line that starts Missing language kotlinforforge version, followed by the version range and the jar that asked for it. Current NeoForge says needs language provider kotlinforforge to load, with no version, when the provider is not installed at all.

1. Stop the server and look in mods/ for a Kotlin for Forge jar. Its file name usually starts with kotlinforforge.
2. Download Kotlin for Forge from its CurseForge or Modrinth page, picking a file listed for your Minecraft version and your loader. Kotlin for Forge publishes different major versions for different Minecraft versions.
3. Choose a version at or above the number in the message. If an older Kotlin for Forge jar is in mods/, take it out, so only one copy remains.
4. Start the server. If the line now names another mod or a higher version, repeat for that one.
5. If the id is neither kotlinforforge nor javafml, it usually belongs to another provider mod: check the requiring mod's page for what it needs.

javafml: a mod built for a newer Forge

javafml is Forge's own language provider. Its version is Forge's major version, which Forge typically raises with each Minecraft version. A mod that asks for a higher javafml than your Forge provides was built for a newer Forge, usually for a newer Minecraft version, and no separate download makes it load. Replace it with the mod's build for your Minecraft version, or remove it.

How to fix it on Fabric

Fabric names the missing dependency by its mod id. fabric-language-kotlin is Fabric Language Kotlin, published on Modrinth and CurseForge.

1. Find the line that names fabric-language-kotlin and note the version after requires version, if there is one.
2. Download Fabric Language Kotlin, picking a file listed for your Minecraft version at or above that version, and put it in mods/.
3. Take out any older copy, so only one remains.
4. Start the server and check that the line is gone.

What MineXHost's launcher does with this

On a Fabric server, when the log says a mod requires fabric-language-kotlin and it is missing, MineXEngine, our launcher, tries to fetch it itself: it looks for a build for your server's loader and Minecraft version on Modrinth or CurseForge, adds the jar to mods/ and restarts the server. It is the same kind of fetch the launcher makes for other missing libraries. If no build can be found, nothing is added, and the steps above apply.

On Forge and NeoForge, crash recovery on a customer server normally names a problem mod and leaves it in place. This error is one of the exceptions. In some cases, when the loader names one jar in a needs language provider … or above line and that jar's own mods.toml asks for exactly that provider and version, the launcher moves the jar to mods/.quarantined/ so the rest of the pack can start, and the console names the jar.

It is a move, not a delete. For kotlinforforge, install the Kotlin for Forge version the message names, move the jar from mods/.quarantined/ back into mods/ and restart. For javafml, put the mod's build for your Minecraft version in mods/ instead.