For Polish, the first scriptures date back to the 9th century.
Source to that?
Later on, in the Middle Ages, the Czech language became very popular in Poland. Almost everyone who belonged to an intellectual circle spoke Czech.
If Czech was that popular in Poland I'm surprised they drifted apart to the point of not being mutually intelligible anymore. Last time Czech was that popular was in 18th century, so only 300+ years?
This created a fashion for the Czech language, which originated in many lexical changes in the Polish language.
For example, Polish words that contain the particle “ra” or “ła” between the consonants, such as “brama” (gate) or “błagać” (to beg), typically originate from the Czech language.
Let me provide more examples of Czech infulunce of Polish:
PL: czerwony Old PL: czerwiony ("red")
PL: wesele Old PL: wiesiele ("wedding reception")
PL: hańba Old PL: gańba ("disgrace")
PL: brama Old PL: brona ("gate")
PL: smutny Old PL: smętny ("sad")
PL: serce Old PL: sierce ("heart")
PL: obywatel Old PL: obywaciel ("citizen")
PL: własny Old PL: włosny ("own")
PL: jedyny Old PL: jedziny ("the only")
PL: rzetelny Old PL: rzecielny ("reliable")
PL: śmiertelny Old PL: śmiercielny ("mortal"/lethal")
PL: obecny Old PL: obiecny ("present"/"current")
PL: swoboda Old PL: świeboda ("freedom"/"liberty")
PL: Wacław Old PL: Więcław (Wenceslaus)
Although Polish and Czech belong to the same subgroup of Slavic languages and share many similarities, they are not mutually intelligible. Linguists claim Czech’s oral intelligibility with Polish is only 36% and written intelligibility 46%.
That's surprising. I'd thought that written intelligibility is even more higher. Btw, if a Pole and a Czech spoke in their languages using basic words and sentences, I'm pretty sure they could understand each other. Literary and colloquial language are two different things.
There are many Czech words/verbs/adjectives/sentences that sound archaic to Poles, but we can still recognize them (thanks to the old literature and scripts). Examples:
CZ: říci PL: rzec ("to say") - still in use in literary language. Nowadays we use
powiedzieć.
CZ: jenom PL: jeno ("only") - this word appears in one verse of our anthem (
Słuchaj jeno bo to nasi biją w tarabany). Nowadays we use
tylko.
CZ: protože PL: przeto że - the word
przecież ("yet"/"though") is somewhat related to this word. Nowadays we use
dlatego or
ponieważ.
CZ: jsem rad PL: jestem rad ("I'm glad") - from
radość ("joy"). Nowadays we use
cieszę się.
CZ: kazit PL: kazić ("to spoil") - nowadays we use
psuć/
zepsuć.
CZ: odpustit PL: odpuścić ("to forgive") - we still use
odpuścić but nowadays it means more like "let it go".
Odpuścić is still used in religious phrases (
odpuszczać grzechy "forgive sins",
odpuść nam nasze winy "forgive us our trespasses"). Nowadays, we simply use
wybaczyć.
CZ: podle PL: podle/
podług ("according to"/"by") - nowadays we use
według.
CZ: spolu PL: społem ("together") -
wspólnie/zespół are related to this, so it's not so alien to us, nowadays we use
razem.
CZ: spojit PL: spoić ("to unite") - nowadays we use
łączyć/
jednoczyć.
CZ: večeře PL: wieczerza ("supper"/"dinner") - we refer
wieczerza only to Christmas Eve Supper, normally we use the Latin loan
kolacja.
CZ: hudba PL: gędźba ("music") - a very forgotten archaism. We use
muzyka nowadays.
CZ: počet PL: poczet ("count"/number") - used more as a collection or set (
Poczet władców Polski "List of Polish rulers"). Nowadays we use
ilość/
liczba.
CZ: poslední PL: pośledni ("last one"/"final") - used still in Silesian dialect, I think, but overall we say
ostatni.
CZ: zkoušet PL: skusić ("to try") - from
pokusa/
zakusa. Nowadays we use
próbować (a loan from Latin).
CZ: hledat PL: ględać ("to search") - nowadays we use
szukać, which itself is a loan from German
suchen (a cognate to English
seek).
CZ: konečně PL: koniecznie ("finally"/"at last") -
koniecznie means nowadays "necessarily" in Polish. Nowadays we use
nareszcie/
wreszcie/
w końcu.
CZ: oběť PL: obiata ("sacrifice"/"victim") - as much forgotten as
żertwa (which means the same thing). Nowadays we use
ofiara (which we borrowed from German O
pfer, which itself comes from Latin
offero). However, the word
obiad ("lunch"/"dinner") is related to this word.
CZ: slíbit/
slibovat PL: ślubować ("to promise") - today it means "to vow"/"pledge"/"to dedicate". Nowadays, we use
obiecać.
CZ: sever PL: siewior ("North") - a very archaic word for a cardinal direction. Nowadays we just use
północ for North (literally "half-night", it's also used for midnight).
CZ: hnědý PL: gniady ("brown") - used for brown colored horse mostly. Nowadays we use
brązowy/
brąz (another loanword).
CZ: modrý PL: modry ("blue") - still used in Silesia, but overall we use now
niebieski.
CZ: jídlo PL: jadło ("food") - an obsolete form of the word
jedzenie, which we use nowadays.
CZ: chuť PL: chuć/
chęć ("taste") -
chęć today means "desire"/"wish". Nowadays we say
smak (a loan from German
Geschmack).
CZ: strýc PL: stryj ("uncle") - today it means paternal uncle, rather than just uncle but even so, the word is mostly unused and will probably be forgotten in a few generations. Nowadays we say
wuj/
wujek.
CZ: milovat PL: miłować ("to love") - used in old-fashioned, traditional and religious phrases mostly. Nowadays we use
kochać.
CZ: ostrov PL: ostrów ("island") - not quite forgotten word but it's rather used for a river island surrounded by watercourse arms (
Ostrów Tumski). Nowadays we just say
wyspa.
CZ: zdroj PL: zdrój ("source") - used for spa towns mostly (
Kudowa-Zdrój,
Lądek-Zdrój, etc.). Nowadays we use
źródło.
CZ: zrcadlo PL: zwierciadło ("mirror") - nowadays more like a fairy-tale word for mirror, although we use
zwierciadło for some optical instruments. Regardless, when it comes to a simple mirror we normally just say
lustro (an Italian loan).
CZ: polévka PL: polewka ("soup") -
Czarna Polewka ("Black Soup") is at least a name of one of our traditional soups, but overall we simply refer to soup as
zupa (from German
Suppe).
CZ: život PL: żywot ("life") - a rather literary/obsolete form of the word
życie, which we use nowadays.
CZ: pouť PL: pąć ("pilgrimage") - at least we occasionally refer to pilgrims as
pątnicy, but overall we use the word
pielgrzymka (from German
Pilgrim, from Latin
peregrinus).
CZ: rychly PL: rychły ("quick"/"fast") - not quite forgotten, but nowadays it sounds rather poetic and it means more like "sudden" or "early". Nowadays we say
szybki.
CZ: řit PL: rzyć ("butt"/"ass"/"anus") - a very archaic and forgotten word for
tyłek/
dupa/
odbyt, which we use nowadays.