Practice is the only way to become proficient in a foreign language.
Literal
Practice [topic-は], foreign-language [locative-に] become-proficient only-way [copula-だ].
A straightforward aphoristic claim. 熟達する ('become skilled at, master') takes に to mark what you're mastering — the に + する pattern is common for 'become proficient at' type verbs (慣れる, 熟達する, 精通する). 唯一の ('the sole, the only') is a high-register modifier.