In Old High German, we find the forms lenzo, längess, also längsing (Middle High German langez), forms which exactly match Old English lencten (cf. also Old Saxonian lentin, Middle Dutch lenten). These attested forms can be derived from a (reconstructed) West Germanic *langa-tīna(z). The meaning of the first element of this compound is clearly 'long'. The second element tīna- is an ancient Indo-European suffix-like formation meaning 'day' or 'daily'. So the word Lenz, lenten most probably refers to the period of the year when daylight is increasing. Literally, we may translate Lenz as '(the season having) long days'.
Now, back to the second element - West Germanic *-tīna: The ancient Indo-European formation is present in the Gothic word sin-teins, which means 'daily'. And it is not only used in old Germanic languages such as English, German, or Gothic, but also in various other old Indo-European languages. In Latin, e.g., we have nūn-dinae, meaning 'the market held every ninth day'.
The suffix is contained in the Old Indian word madhyam-dina ''midday', 'time of midday'. The Lithuanian language as well as Slavonic languages have diena (Lithuanian) and dini (Old Church Slavonic).
It is clear that our suffix-like element -tina or -dina is derived from the Proto-Indo-European base *dyeu- (cf. Skt. diva 'by day', Latin dies, Welsh diw, Arm. tiw), which has developed so many meanings, from 'sky' to 'day' to 'heaven' to 'god'.
Our compound word *langa-teina, Lenz, lenten must originally have been an adjective, just because of its Bahuvrīhi character. A Bahuvrīhi-compound always has the meaning of 'possessing' the things/features mentioned, such as Ironheart means 'having or possessing a heart of iron'; Bahuvrīhis are always adjectives originally.
So, as to be expected, in the Old High German phrase lengizin manoth (meaning 'the month having long days' for the month of March) lengizin is clearly an adjective.
In Modern English Lent or Lenten has acquired a specialized meaning. The Church sense of the period between Ash Wednesday and Easter is peculiar to English.
