By Denis Dyomkin and Timothy Heritage ST PETERSBURG, Russia (Reuters) - President Vladimir Putin boasted on Friday that Russia had found the "inner strength" to prevent sanctions causing a deep economic crisis, and told the West to stop using "the language of ultimatums." Investment in Russia has slowed to a trickle, capital flight has risen and the economy has been sliding into recession since oil prices tumbled last year and the West imposed economic sanctions on Moscow over the Ukraine crisis.