The word myriad [ˈmɪɹi.æd/ˈmɪɹi.əd] means a countless number or multitude, and in the past it meant 10,000. It comes from the French myriade (myriad, 10,000), from the Latin Latin myrias (10,000), from the Ancient Greek μυριάς (muriás – countless, 10,000), from μῡρῐ́ος (mūríos – numberless, countless, infinite) [source].
The use of 10,000 to mean countless or infinite happens in other languages as well. For example in Chinese 万 [萬] (wàn) means 10,000 or a great number [source]. The same character 万 (man) in Japanese means 10,000, a myriad, everything, all or various. When pronounced ban it means completely, absolutely or totally [source].
Do other languages do something similar?
Other English words that refer to a large but unspecified number include um(p)teen or umpty, which come from umpty (a colloquial name for a dash in Morse Code used as World War I army slang) and -teen [source].
Also zillion, gazillion, bazillion, jillion, bajillion and squillion [source].
Do you have any others?