As for Esperanto, エスペラント and エスペラント語 are used interchangeably. It's sufficient to just say エスペラント when the listeners know it's the name of a language. However, for articles intended for people less familiar with Esperanto, such as general newspaper articles, it would be definitely helpful to say エスペラント語. This 5-minute tour created by the Japanese Esperanto Institution uses エスペラント語, which likely is intentional.
I think the same is basically true for Toki Pona, but the Wikipedia article for トキポナ explicitly says this:
(Since "toki" itself means "language", saying トキポナ語 is redundant.)
So if you are seriously studying Toki Pona, it would be better to say トキポナ following this advice. However, I don't think it's wrong to say トキポナ語 in articles for the general public, and I can find many examples of トキポナ語 on the net. After all, we already have many examples similar to this, such as リオグランデ川 (River Great River; Rio Grande), サハラ砂漠 (Desert Desert; Sahara), and モンブラン山 (Mountain White Mountain; Mont Blanc). English speakers also say "Mount Fujiyama", "LCD display", etc. These are technically redundant but are helpful to many people.