diff --git a/hornbeam_interpreter/src/engine.rs b/hornbeam_interpreter/src/engine.rs
index 9ec8ab4..e4c8091 100644
--- a/hornbeam_interpreter/src/engine.rs
+++ b/hornbeam_interpreter/src/engine.rs
@@ -40,6 +40,18 @@ pub enum Value {
     Reflective(Box<dyn Reflect>),
 }
 
+impl Value {
+    pub fn get_type_name(&self) -> String {
+        String::from(match self {
+            Value::Str(_) => "Str",
+            Value::Int(_) => "Int",
+            Value::Bool(_) => "Bool",
+            Value::List(_) => "List",
+            Value::Reflective(reflective) => reflective.type_name(),
+        })
+    }
+}
+
 lazy_static! {
     static ref U8_TYPEID: TypeId = TypeId::of::<u8>();
     static ref U16_TYPEID: TypeId = TypeId::of::<u16>();
diff --git a/hornbeam_interpreter/src/localisation/fluent.rs b/hornbeam_interpreter/src/localisation/fluent.rs
index ac0708e..cd5d74b 100644
--- a/hornbeam_interpreter/src/localisation/fluent.rs
+++ b/hornbeam_interpreter/src/localisation/fluent.rs
@@ -4,19 +4,81 @@
 
 use crate::engine::Value;
 use crate::interface::LocalisationSystem;
+use fluent_templates::fluent_bundle::types::FluentNumber;
+use fluent_templates::fluent_bundle::FluentValue;
+use fluent_templates::loader::langid;
+use fluent_templates::{ArcLoader, LanguageIdentifier, Loader};
 use std::borrow::Cow;
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, HashMap};
 
-pub struct FluentLocalisationSystem {}
+use thiserror::Error;
+
+fn interpreter_value_to_fluent_value(v: &Value) -> Option<FluentValue> {
+    match v {
+        Value::Str(str) => Some(FluentValue::String(Cow::Borrowed(&str))),
+        Value::Int(int) => {
+            // This is an unstyled number
+            // TODO Support fancier numbers
+            Some(FluentValue::Number(FluentNumber::from(int)))
+        }
+        Value::Bool(_) => {
+            // TODO I can't believe there's no way to expose this to fluent!
+            None
+        }
+        Value::List(_) => None,
+        Value::Reflective(_) => None,
+    }
+    // TODO It might be good to support a `struct` type here with fancier numbers
+    //      or to support FluentNumber types directly, then expose functions to Hornbeam
+    //      templates to create them?
+}
+
+pub struct FluentLocalisationSystem {
+    pub fluent: ArcLoader,
+}
+
+impl FluentLocalisationSystem {}
+
+#[derive(Error, Clone, Debug)]
+pub enum FluentLocalisationError {
+    #[error("no translation found for {trans_key} in {lang_id}")]
+    NoTranslation { trans_key: String, lang_id: String },
+
+    #[error("can't convert param {param_name} of type {param_type} to a fluent type")]
+    IncompatibleValue {
+        param_name: String,
+        param_type: String,
+    },
+}
 
 impl LocalisationSystem for FluentLocalisationSystem {
-    type Error = ();
+    type Error = FluentLocalisationError;
 
     fn lookup(
         &self,
         trans_key: &str,
         params: &BTreeMap<&str, Value>,
     ) -> Result<Cow<str>, Self::Error> {
-        todo!()
+        let li: LanguageIdentifier = langid!("en-GB"); // TODO get the language tag from somewhere
+        let mut mapped_params = HashMap::with_capacity(params.len());
+        for (&k, v) in params {
+            if let Some(mapped_val) = interpreter_value_to_fluent_value(v) {
+                mapped_params.insert(k, mapped_val);
+            } else {
+                return Err(FluentLocalisationError::IncompatibleValue {
+                    param_name: String::from(k),
+                    param_type: v.get_type_name(),
+                });
+            }
+        }
+        match self.fluent.lookup_with_args(&li, trans_key, &mapped_params) {
+            Some(val) => Ok(Cow::Owned(val)),
+            None => {
+                return Err(FluentLocalisationError::NoTranslation {
+                    trans_key: String::from(trans_key),
+                    lang_id: li.to_string(),
+                });
+            }
+        }
     }
 }