Many words in English are actually "borrowed" from other languages. If the word is transliterated from a different alphabet (like Polish), the English spelling is meant to approximate the pronunciation in the original language, but the actual pronunciation is meant to be as close as an English speaker can manage to the original.
There are hundreds of such words — for example, the sauce catsup/ketchup is from the Chinese 茄汁, pronounced more like "ke-jap". Over time (sometimes very quickly) the pronunciation is anglicized to something native speakers can easily pronounce. Another example, karaoke, is borrowed from the Japanese, who pronounce it as "kah- rah- oh- keh", while many Americans pronounce it more like "carree- okee".
(As a side note, the "oke" in "karaoke" is actually an abbreviation of the English word "orchestra", which the Japanese themselves borrowed. It goes around and around.)
Lechitic itself comes from Polish:
The term Lechitic derives from the most popular form of the name of the legendary forefather of Poland, Lech (apparently a distorted form of *lęch). Furthermore, Lechites (Polish: Lechici) was an ethnic and linguistic group of West Slavs, the ancestors of modern Poles and the historical Pomeranians and Polabians. source
I think the proper pronunciation of "lech" would be as close to the original Polish as possible, with the "itic" pronounced the same as in "Semitic" or "Levitic". If this video of (what I assume is) a native Polish speaker pronouncing "Lech Walesa" is accurate, "Lech" is pronounced with a "hard" ch sound, like chemist or chorus. Therefore Lechitic is likely pronounced as "lek- i- itic" — with some possible, subtle variations.