In みんなの日本語初級I book, the word 「ことば」 is used to mean "language" (English, Japanese, Russian, etc.)
However, I found 「言語」 as the most proper word for this type of language. The first variant (which is used in the book) is more for some abstract language.
Why is 「ことば」 used here instead?
言語 is more of an academic term, while ことば is more colloquial and accessible. 言語 is normally used with longer compound words. Functionally, though, they mean the same thing.
Although the example that you posted is technically academic, the use of furigana does indicate that it is designed either for younger Japanese or for non-native speakers. Hence, ことば is more accessible (as @tarkma has pointed out).
In addition to tarkma's and BJCUAI's answers, I'd like to draw your attention to the header:
国【くに】・人【ひと】・ことば
All three of these are so-called 大和【やまと】言葉【ことば】, or native-Japanese terms, which are read with the 訓読【くんよ】み. The word 言語【げんご】, meanwhile, is read with the 音読【おんよ】み, and originates as a borrowing from Chinese. For a consistently 音読【おんよ】み header of the same meaning, we could say instead:
国家【こっか】・国民【こくみん】・言語【げんご】
However, this comes across very differently.
The terms native to the language have a more informal, familiar, and comfortable sense, while the terms borrowed from Chinese tend to be more formal, academic, and sometimes stilted. This is similar in some ways to English vocabulary, where words native to English are more informal, familiar, and comfortable, while the terms borrowed from Latin tend to be more formal, academic, and sometimes stilted.
I think the word ことば is used here simply because it is an easier word. In the Japanese education system, the word 言語 is introduced in second grade according to this.
