The Kingdom of Bohemia was rather an independent entity within the HRE. It had its own dynasty, Premyslid, which ruled Bohemia since 872/875 onward, and had own administration independent of the Empire. Bohemia was permanently incorporated into the HRE in 1002-3 during the regency of Queen Emma. Bohemia until 1200 played a minor role in the HRE as it was not developed as the wealthier provinces in the west and south of it. The nobility until 1627 was predominantly of the Czech/Slavic origin as many came either from archaic tribal units or from the administrations of the castle district founded during the reign of the Boleslav Cruel (+972). The castle districts formed the executive power of Bohemia and Moravia, and where Bohemian dukes and kings were giving them under the control of the trusted advisors. The castles than became the foundation of the future feudal land holdings and the administration moved in the 13th century to the cities. If you read an extensive dictionary and multi-volume books Hrady, zámky a tvrze Království českého (Castles, chateaus and manors of the Bohemian kingdom), you can read about the ownership of any castles and land holdings from the earliest parts of the Czech history.

The German ethnicity did not start pushing into the Bohemia until the 13th century. While there were German, Frankish communities within Bohemia and Moravia already in the 9th century, a wide resettling did not start until with the development of the cities and mining industry. The first cities, or municipalities with city rights appeared the first time around 1210-1220 (Uničov, Bruntál, Opava, Znojmo, Jemnice, Hodonín, Hradec Králové, Litoměřice). Large agglomeration like Prague, Brno and Olomouc existed for centuries, but they did not get their city rights until 1230s-1240s. The German migration wave peaked in Moravia in the middle of the 13th century due its devastation caused by Mongols under the leadership of Bishop Bruno of Schaumburk. In Bohemia, the migration from the west continued with the mining boom in cities like Kutna Hora until the 14th century. Cities were quickly Germanized due laws and businesses, but even in the 14th century, we cannot say that Prague was a German city. Various analyses of the tax records, guilds, and church records due point out on a diverse national makeup (for example, there were a lot of French centered in area around St. Linhard in Prague). Prague New Town was founded as a dominant Czech city. Demography of the cities changed in the late 14th century when the Black Death caused resettling from within the Czech core. The decades prior and after the Hussite Revolution, the leading cities (Prague, Hradec Kralove, Plzen, Kolin, Kutna Hora) in Bohemia were only Czech speaking. The share of Germans diminished even in Moravia where cities like Odry, Mohelnice, Zahreb and Litovel became Hussite towns.

The aristocracy under the Premyslid and Luxembourg dynasties generally Czech speaking. It was hard to get accepted into their ranks as a foreigner, but it happened (Arnold von Hückeswagen from Rhineland built one of the largest castles in Moravia, Hukvaldy). Also, it is important to point out, that the ruling elite in Bohemia was multicultural, where families were interconnected from France and Italy to Kievan Rus. The aristocracy at the apex mimic the multicultural fabric of its royal dynasty. Bohemia achieved the peak of the power and position within the HRE under the reign of the Charles IV, but the Hussite War weakened the bond between the empire and the Czech Lands.

The Czech language only played a role within the Czech territories, not outside the kingdom. In the 15th century, the country was isolated and it was forbidden to trade with heretics. Nuremberg did not start trading with Prague again until 1496. Between 1419 and 1500s, the trade between Bohemia and the Empire was conducted by the Catholic cities like Pilsen and Budejovice. Pilsen became from this arraignment as the most important city in Bohemia after Prague. The Hussite War and Bohemian Reformation in the 15th century enforced that the Czech language would be only one used in the church rite, masses, and within the administration. The knowledge of the German language went down and was not discovered again until the Renaissance.

The significant event that pushed the Czech language into the secondary role was the Battle of White Mountain in 1620 and elevating the German language as another official language of the kingdom in 1627. Until the end of the 17th century, the Czech language was dominant even within the Catholic church, but its use diminished with each subsequent generations. The Germanization peaked in the 18th century when only a small segment of the literate people could use it. The Germanization effort was lead from two sources, from the administration of Joseph II and from the Catholic church, where the important church institutions, collegiate churches, abbeys, schools, lyceums, and parishes became dominated by the German language. Therefore, the Austrian German society started to see the Czech language as the language of the peasants and urban poor.

Please note that your question is very broad. KoB was part of the HRE for many centuries with the importance of the Czech people and usage of the Czech language changing. It was different under the Czech Premyslid dynasty, it was different under the Luxembourg dynasty, the Jagiellonian dynasty and definitely different under the Austrian Habsburg dynasty.

Probably the only uniting aspect was that Czechs in the countryside of KoB spoke Czech. Not necessarily wrote; just spoke.

It is also important to note that not all inhabitants of KoB were Czechs. Some Bohemian rulers, even rulers of the Czech Premyslid dynasty, felt it was practical to invite German settlers in order to improve population level as well as to improve craftsmanship level. Therefore not even in the countryside was the KoB population homogenous in spoken language, although German speakers were more concentrated in cities, towns and industrial settlements.

Czech language spread beyond KoB borders in one specific era though; when Bohemian rulers were also elected rulers of the HRE.

Charles IV. of Luxembourg, King of Bohemia (1346-1378) and Holy Roman Emperor (1355-1378) issued the Golden Bull (1356), where he, among other decisions, set the Czech language (article XXXI.) as one of the three official languages of the Empire (together with German and Italian), promoted literature written in Czech and initiated the creation of Klaret's vocabularies. He sponsored the foundation of the Emmaus Monastery in Prague and its scriptorium, where numerous texts in Old Slavonic were created.

The Golden Bull as a whole remained in effect until the demise of the HRE in 1806. The article XXXI. itself was treated differently by individual rulers. Even during the rule of Charles IV. himself it wasn't strictly demanded in all cases.

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Source in Czech:

Jiří Hasil (Bohemistic Studies Institute, Charles University): Charles IV. and the Czech language, 2017