The nationality words that you have learned in the previous lessons can be used to name languages.

You have to take a nationality, remove its ending and add the suffix -iškai:

lietuvis - is + iškai = lietuviškai
anglas - as + iškai = angliškai
ispanas - as + iškai = ispaniškai

Single exception: vokietis - ietis + iškai = vokiškai

These new words are adverbs. They literally mean "in Lithuanian", "in English", "in Spanish", "in German".

Adverbs go naturally with verbs:

Aš kalbu lietuviškai. I speak (in) Lithuanian.
Amerikietis kalba angliškai. An American speaks (in) English.
Argentinoje mes kalbame ispaniškai. In Argentina we speak (in) Spanish.

If you use the noun "kalba" (which means "language") and the genitive plural case of a nation, you can then name a language:

lietuvių kalba = the Lithuanian language
anglų kalba = the English language
prancūzų kalba = French

Meksikos kalba yra ispanų kalba. The language of Mexico is Spanish.
Brazilijoje oficiali kalba yra portugalų kalba. In Brazil the official language is Portuguese.
Man patinka arabų kalba. I like Arabic.