What's the best database structure to keep multilingual data?

Similar to method 3:

[languages]
id (int PK)
code (varchar)

[products]
id (int PK)
neutral_fields (mixed)

[products_t]
id (int FK)
language (int FK)
translated_fields (mixed)
PRIMARY KEY: id,language

So for each table, make another table (in my case with "_t" suffix) which holds the translated fields. When you SELECT * FROM products, simply ... LEFT JOIN products_t ON products_t.id = products.id AND products_t.language = CURRENT_LANGUAGE.

Not that hard, and keeps you free from headaches.

Your third example is actually the way the problem is usually solved. Hard, but doable.

Remove the reference to product from the translation table and put a reference to translation where you need it (the other way around).

[ products ]
id (INT)
price (DECIMAL)
title_translation_id (INT, FK)

[ translation ]
id (INT, PK)
neutral_text (VARCHAR)
-- other properties that may be useful (date, creator etc.)

[ translation_text ]
translation_id (INT, FK)
language_id (INT, FK) 
text (VARCHAR)

As an alternative (not especially a good one) you can have one single field and keep all translations there merged together (as XML, for example).

<translation>
  <en>Supplier</en>
  <de>Lieferant</de>
  <fr>Fournisseur</fr>
</translation>

In order to reduce the number of JOIN's, you could keep separate the translated and non translated in 2 separate tables :

[ products ]
id (INT)
price (DECIMAL)

[ products_i18n ]
id (INT)
name (VARCHAR)
description (VARCHAR)
lang_code (CHAR(5))