I18n.format is not necessary...
and if a variable is null, then it instanceof MyClass is always false.
if(potioneffect.getAmplifier() == 0) s1 = s1 + " I";
else if (potioneffect.getAmplifier() == 1) s1 = s1 + " II";
else if (potioneffect.getAmplifier() == 2) s1 = s1 + " III";
else if (potioneffect.getAmplifier() == 3) s1 = s1 + " IV";
else if (potioneffect.getAmplifier() == 4) s1 = s1 + " V";
else if (potioneffect.getAmplifier() == 5) s1 = s1 + " VI";
else if (potioneffect.getAmplifier() == 6) s1 = s1 + " VII";
else if (potioneffect.getAmplifier() == 7) s1 = s1 + " VIII";
else if (potioneffect.getAmplifier() == 8) s1 = s1 + " IX";
else if (potioneffect.getAmplifier() == 9) s1 = s1 + " X";
else if (potioneffect.getAmplifier() >= 10) s1 = s1 + " X+";
else s1 = s1 + "¿";
in this part I think it's better to use switch
