2. Read Languages:

At 4th level (Robber), thieves are able to read 20% of languages,
and this ability increases by 5% with each
additional level of experience until an 80% probability is
attained. This enables the possible reading of instructions and
treasure maps without having to resort to a magic item or
spell.
 

Reading Languages can be attempted but once.

Failure - a score greater
than the percentage chance shown for the experience level of the thief -
<make intelligible> means that the language is not understood and no amount of reading will
make it comprehensible. <make intelligible>
However, the thief can try again with each new experience level he or she gains.

DMG:

Read languages:
This ability assumes that the language is, in fact, one which the thief has encountered sometime in the past.
Ancient and strange languages (those you, as DM, have previously designated as such) are always totally unreadable.
Even if able to read a language,
the thief should be allowed only to get about that percentage of the meaning of what is written
    as his or her percentage ability to read the tongue in the first place.
The rest they will have to guess at.
Languages which are relatively close to those known by the thief will not incur such a penalty.