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In what language is one to count? [1]
One may count in any language that he understands. One may not count in a language that he does not understand, even if it is in Lashon Hakodesh. [Thus, if he does not understand the Hebrew counting then he may not count in Hebrew.] If one counted in a language that he does not understand, even if he counted in Lashon Hakodesh, he does not fulfill his obligation [and must recount with a blessing].[2]
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[1] Admur 489/10; M”A 489/2
Other opinions: Some Poskim rule one fulfills his obligation even if he does not understand the words he is counting and even if it is not in Lashon Hakodesh. [Siddur Yaavetz Halacha 10; Sheilas Yaavetz 139]
[2] The reason: As since he does not know the number that he is counting it is therefore not considered a counting at all. [ibid]