Refers to the French language. Often used in contexts like 仏語を話す (speak French) or 仏語の新聞 (French-language newspaper).
He is fluent in French.
I'm taking a French class.
This letter is written in French.
He speaks French fluently.
I bought a French-language newspaper in front of the station.
仏語 is the general term for the French language itself, while 仏文 specifically points to written French or French literary works, often in academic contexts.
仏語 refers to French while 独語 refers to German; both are kanji abbreviations built from the first character of the country's name, so learners can mix up which kanji stands for which country.
Abbreviation of フランス語 (Furansu-go), using the kanji 仏 (Futsu) as an ateji for 'France'.