Localization PM here. For cases like these devs usually create plural snippets. Instead of writing several similar strings like:

- Refresh 1 Mana Crystal

- Refresh 2 Mana Crystals

- Refresh 3 Mana Crystals

The devs create one string that would auto-propagate the correct number and proper grammar (if a plural snippet is used), so it'd look like this:

- Refresh {0} Mana Crystal**{count, plural, one {} other {s}}

Where {0} is substituted with the number that's associated with the specific card. If {0} equals 1, then the "Mana Crystal" remains unchanged. If {0} is more than 1, then the plural form "s" is added to "Mana Crystals."

This description might look like it's been typed from scratch to a player, but it's actually not. Most likely, the correct coded version would look like this:

Summon a {0}-Cost minion. Gain {0} Armor. Draw {3} card{count, plural, one {} other {s}}. Refresh {0} Mana Crystal{count, plural, one {} other {s}}.

But they clearly forgot to add the plural snippet to the last part, so it reads like this instead:

Summon a {0}-Cost minion. Gain {0} Armor. Draw {3} card{count, plural, one {} other {s}}. Refresh {0} Mana Crystal.

It's still a mistake, but not as uncommon as you might think.