Machine Translation

The Problem

A model reads a sentence in one language and produces a sentence in another. Length varies. Word order varies. Some source words map to multiple target words and vice versa. Idioms refuse one-to-one mapping. "I miss you" in French is "tu me manques" — literally "you are lacking to me." No word-level alignment survives that.

Machine translation is the task that forced NLP to invent encoder-decoders, attention, transformers, and eventually the whole LLM paradigm. Every step forward arrived because translation quality was measurable and the gap between human and machine was stubborn.

This lesson skips the history lesson and teaches the working pipeline of 2026: pretrained multilingual encoder-decoder (NLLB-200 or mBART), subword tokenization, beam search, BLEU and chrF evaluation, and the handful of failure modes that still ship to production uncaught.

The Concept

Modern MT is a transformer encoder-decoder trained on parallel text. The encoder reads the source in its language's tokenization. The decoder generates the target, one subword at a time, using the encoder's output via cross-attention (lesson 10). Decoding uses beam search to avoid the greedy-decoding trap. The output is detokenized, detruecased, and scored against a reference.

Three operational choices drive real-world MT quality.

• Tokenizer. SentencePiece BPE trained on a mixed-language corpus. Shared vocabulary across languages is what enables zero-shot pairs in NLLB.
• Model size. NLLB-200 distilled 600M fits on a laptop. NLLB-200 3.3B is the published production default. 54.5B is the research ceiling.
• Decoding. Beam width 4-5 for general content. Length penalty to avoid too-short output. Constrained decoding when you need terminology consistency.

seq2seq-alignment

Build It

Step 1: a pretrained MT call

from transformers import AutoTokenizer, AutoModelForSeq2SeqLM

model_id = "facebook/nllb-200-distilled-600M"
tok = AutoTokenizer.from_pretrained(model_id, src_lang="eng_Latn")
model = AutoModelForSeq2SeqLM.from_pretrained(model_id)

src = "The cats are running."
inputs = tok(src, return_tensors="pt")

out = model.generate(
    **inputs,
    forced_bos_token_id=tok.convert_tokens_to_ids("fra_Latn"),
    num_beams=5,
    length_penalty=1.0,
    max_new_tokens=64,
)
print(tok.batch_decode(out, skip_special_tokens=True)[0])

Les chats courent.

Three things matter here. src_lang tells the tokenizer which script and segmentation to apply. forced_bos_token_id tells the decoder which language to generate. Both are NLLB-specific tricks; mBART and M2M-100 use their own conventions and they are not interchangeable.

Step 2: BLEU and chrF

BLEU measures n-gram overlap between output and reference. Four reference n-gram sizes (1-4), geometric mean of precisions, brevity penalty for too-short output. The score is in [0, 100]. Commonly used. Frustrating to interpret: 30 BLEU is "usable"; 40 is "good"; 50 is "exceptional"; differences under 1 BLEU are noise.

chrF measures character-level F-score. More sensitive to morphologically rich languages where BLEU undercounts matches. Often reported alongside BLEU.

import sacrebleu

hypotheses = ["Les chats courent."]
references = [["Les chats courent."]]

bleu = sacrebleu.corpus_bleu(hypotheses, references)
chrf = sacrebleu.corpus_chrf(hypotheses, references)
print(f"BLEU: {bleu.score:.1f}  chrF: {chrf.score:.1f}")

Always use sacrebleu. It normalizes tokenization so scores are comparable across papers. Rolling your own BLEU computation is how misleading benchmarks happen.

The three-tier evaluation hierarchy (2026)

Modern MT evaluation uses three complementary metric families. Ship with at least two.

• Heuristic (BLEU, chrF). Fast, reference-based, interpretable, insensitive to paraphrase. Use for legacy comparison and regression detection.
• Learned (COMET, BLEURT, BERTScore). Neural models trained on human judgment; compare semantic similarity of translation to source and reference. COMET has the highest association with MT research since 2023 and is the 2026 production default where quality matters.
• LLM-as-judge (reference-free). Prompt a large model to score translations on fluency, adequacy, tone, cultural appropriateness. GPT-4-as-judge matches human agreement ~80% of the time when the rubric is well designed. Use for open-ended content where no reference exists.

Practical 2026 stack: sacrebleu for BLEU and chrF, unbabel-comet for COMET, and a prompted LLM for the final human-facing signal. Calibrate every metric against 50-100 human-labeled examples before trusting it on production data.

Reference-free metrics (COMET-QE, BLEURT-QE, LLM-as-judge) let you evaluate translations without a reference, which matters for long-tail language pairs where reference translations do not exist.

Step 3: what breaks in production

The working pipeline above will translate fluently 80% of the time and silently fail the remaining 20%. Named failure modes:

• Hallucination. Model invents content that was not in the source. Common in unfamiliar domain vocabulary. Symptom: output is fluent but claims facts the source did not state. Mitigation: constrained decoding on domain terms, human review on regulated content, monitoring for output much longer than input.
• Off-target generation. Model translates into the wrong language. NLLB is surprisingly prone to this on rare language pairs. Mitigation: verify forced_bos_token_id and always decode with a language-ID model check on output.
• Terminology drift. "Sign up" becomes "s'inscrire" in doc 1 and "créer un compte" in doc 2. For UI text and user-facing strings, consistency matters more than raw quality. Mitigation: glossary-constrained decoding or post-edit dictionary.
• Formality mismatch. French "tu" vs "vous", Japanese politeness levels. The model picks whichever form was more common in training. For customer-facing content this is usually wrong. Mitigation: prompt prefix with a formality token if the model supports it, or fine-tune a small model on formal-only corpora.
• Length explosion on short input. Very short input sentences often produce overlong translations because the length penalty falls off a cliff below ~5 source tokens. Mitigation: hard max-length cap proportional to source length.

Step 4: fine-tuning for a domain

Pretrained models are generalists. Legal, medical, or game-dialog translation benefits measurably from fine-tuning on domain parallel data. The recipe is not exotic:

from transformers import Trainer, TrainingArguments
from datasets import Dataset

pairs = [
    {"src": "The defendant pleaded guilty.", "tgt": "L'accusé a plaidé coupable."},
]

ds = Dataset.from_list(pairs)


def preprocess(ex):
    return tok(
        ex["src"],
        text_target=ex["tgt"],
        truncation=True,
        max_length=128,
        padding="max_length",
    )


ds = ds.map(preprocess, remove_columns=["src", "tgt"])

args = TrainingArguments(output_dir="out", per_device_train_batch_size=4, num_train_epochs=3, learning_rate=3e-5)
Trainer(model=model, args=args, train_dataset=ds).train()

A few thousand high-quality parallel examples beats a few hundred thousand noisy web-scraped ones. Quality of training data is the single largest production lever.

Use It

The 2026 production stack for MT:

LLMs now outperform specialized MT models on several language pairs as of 2026, particularly on idiomatic content and long context. The tradeoff is per-token cost and latency. Pick an LLM when context length, stylistic consistency, or domain adaptation via prompting matters more than throughput.

Ship It

Save as outputs/skill-mt-evaluator.md:

---
name: mt-evaluator
description: Evaluate a machine translation output for shipping.
version: 1.0.0
phase: 5
lesson: 11
tags: [nlp, translation, evaluation]
---

Given a source text and a candidate translation, output:

1. Automatic score estimate. BLEU and chrF ranges you would expect. State whether a reference is available.
2. Five-point human-verifiable check list: (a) content preservation (no hallucinations), (b) correct language, (c) register / formality match, (d) terminology consistency with glossary if provided, (e) no truncation or length explosion.
3. One domain-specific issue to probe. E.g., for legal: named entities and statute citations. For medical: drug names and dosages. For UI: placeholder variables `{name}`.
4. Confidence flag. "Ship" / "Ship with review" / "Do not ship". Tie to the severity of issues found in step 2.

Refuse to ship a translation without a language-ID check on output. Refuse to evaluate without a reference unless the user explicitly opts in to reference-free scoring (COMET-QE, BLEURT-QE). Flag any content over 1000 tokens as likely needing chunked translation.

Exercises

1. Easy. Translate a 5-sentence English paragraph to French and back to English using nllb-200-distilled-600M. Measure how close the round-trip is to the original. You should see semantic preservation with word-choice drift.
2. Medium. Implement a language-ID check on translation outputs using fasttext lid.176 or langdetect. Integrate into the MT call so off-target generations are caught before returning.
3. Hard. Fine-tune nllb-200-distilled-600M on a 5,000-pair domain corpus of your choice. Measure BLEU on a held-out set before and after fine-tuning. Report which kinds of sentences improved and which regressed.

Key Terms

Further Reading

• Costa-jussà et al. (2022). No Language Left Behind: Scaling Human-Centered Machine Translation — the NLLB paper.
• Post (2018). A Call for Clarity in Reporting BLEU Scores — why sacrebleu is the only correct way to report BLEU.
• Popović (2015). chrF: character n-gram F-score for automatic MT evaluation — the chrF paper.
• Hugging Face MT guide — practical fine-tuning walkthrough.