Read languages is one of those spells that really depends on dungeon and setting design to be useful. If there aren't any messages to be read, nobody will bother to memorize it, let alone cast it, but it's the sort of spell that can really let the party magic-user shine beyond (or without) simply being a trump card in combat situations, as well as add tremendous depth to a campaign.
Real-world archaeologists and treasure-hunters (not to mention spies, merchants, minstrels, travelers, historians, and other scholarly sorts) throughout the ages would probably have sold their own grandmothers into slavery for such a versatile translation tool. Dead languages, foreign tongues, heiroglyphs, pictograms, and codes and ciphers are all fertile targets for a humble 1st-level mage with a read languages spell at his disposal. If you use sci-fi elements in your D&D game, throw in alien and computer languages, too.
Unfortunately, few published adventures or settings give such things a second thought, or even a first, which is frankly a damn shame. It's up to you, as DM, to see that your setting (whether consisting of published material or homebrew or some combination) is stocked with these elements. Here are some possibilities for a typical fantasy RPG world.
Signs, inscriptions, plaques, graffiti, books, scrolls, tapestries, slates or tablets, coins, weapons and armor, etchings, murals, maps, labels, and other items may bear messages in unfamiliar languages.
They could have been placed by the original builders or occupants of the dungeon, carried in by monsters as incidental treasure or clutter, or brought in or written by previous explorers.
Information conveyed could be names or functions of rooms, instructions to operate dungeon machinery, directions, warnings, historical records, legends, prophecies, prayers, rituals, manifests of goods, epitaphs, dedications, curses (the non-magical kind,) bawdy jokes, riddles, mottos and slogans, wills, last words, exploration notes, property deeds, genealogies, ransom demands, personal communications, monograms, recipes, plans and blueprints ...
While a few can be useless or just for laughs, most should be either helpful or informative, so as not to discourage players from reading them.
Of course, not all of these things have to be written in ancient or foreign tongues. It's good to be consistent, though: if the glyphs in one room of the dungeon are in ancient Tharbanic, other features built around the same time should be too. Missives recorded in newer tongues represent different ages and circumstances, lending some verisimilitude to the setting and at the same time mixing things up a bit for the players.
As a final note, it may be desirable to nerf the thief's standard read languages ability to affect only coded messages and the like, rather than all unfamiliar languages; otherwise, once the party has a 4th-level thief, the magic-user spell again becomes largely superfluous.
