From Old English glaedene, "gladden" is another name for the "flag" or "iris", now usually spelt gladdon. Tolkien suggested to translators to avoid if possible the 'learned' name iris.
Noldorin
palath-ledin
place name. Gladden Fields
Sindarin
loeg ningloron
place name. Gladden Fields
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
English
Gladden Fields
Gladden Fields
Earliest Elvish name for the Gladden Fields appearing in Lord of the Rings drafts from the 1940s (TI/114), a combination of palath “iris” and a plural (possibly Ilkorin) form of lhad “plain”, as suggested by Roman Rausch (EE/2.8).