Why would a Spanish Catholic priest disapprove of German Catholic texts for Gloria and Sanctus?

In 2001 the Vatican published Liturgiam Authenticam.

This considered the experience of the Church since the Second Vatican Council encouraged the celebration of Mass in the vernacular languages of the people, rather than in Latin. It expressed the view that some translations into vernacular languages had been too free and were, in parts, adaptions rather than strict translations.

For example, section 6 includes the following:

Nevertheless, it has been noted that translations of liturgical texts in various localities stand in need of improvement through correction or through a new draft. The omissions or errors which affect certain existing vernacular translations – especially in the case of certain languages – have impeded the progress of the inculturation that actually should have taken place.

The reference here to "certain languages" is widely taken to include both German and English.

Section Twenty goes on:

it is to be kept in mind from the beginning that the translation of the liturgical texts of the Roman Liturgy is not so much a work of creative innovation as it is of rendering the original texts faithfully and accurately into the vernacular language. While it is permissible to arrange the wording, the syntax and the style in such a way as to prepare a flowing vernacular text suitable to the rhythm of popular prayer, the original text, insofar as possible, must be translated integrally and in the most exact manner, without omissions or additions in terms of their content, and without paraphrases or glosses. Any adaptation to the characteristics or the nature of the various vernacular languages is to be sober and discreet.

This emphasises the importance of sticking closely to the original text. Original does not, here, mean the ancient text, it means the text on which the vernacular translation is supposed to be based, which will be the Latin text of the latest Roman Typical Edition. It also says some variation in wording, syntax and style is permissible.

So there is a tension about how far or near a translation can legitimately be. This may explain why a Spanish Catholic priest might disapprove of some of the variations allowed in the German Mass.

One reason why the Spanish Mass sticks more closely to the Latin is that Spanish, like Portuguese, Italian and French, is a Romance language, which means it evolved from Latin in the first place. The common structure makes translation from Latin to Spanish much simpler, and more precise and natural, than translating Latin to a Germanic language, such as German or English.

A second reason is that Germany has a strong choral tradition. Musical settings of such texts as the Sanctus or Gloria, originally in Latin, can be sung in German but will require the choice of German words to be influenced by the number of syllables etc. needed to fit the music. While this will have been done, presumably, without significantly affecting the meaning, and will anyway have been accepted by Rome, this Spanish priest may feel strongly that such adaptations are essentially corruptions of the Latin.

Thirdly Germany, like England, has large numbers of liturgical Protestants. The Lutheran and Anglican services also have translations of the Sanctus and Gloria and the ecumenical desire for a common text was perhaps a factor in German. It was so in English, where Sabaoth was translated "power and might" rather than "hosts" in 1973 but "corrected" in 2011. (Even St Paul did not translate the Hebrew word sabaoth into Greek e.g.Romans 9 29). Linking back to the second point, musical settings originally written for Lutheran worship, and therefore in the German language, may be suitable to Catholic worship (or not, as the view of your Spanish priest may be).

The German Catholic bishops have taken a more robust line in resisting changes following Liturgiam Authenticam, than the English-speaking bishops did. (See here.) Also here is an article about the refusal of the German and Austrian bishops, in 2018, to change the "lead us not into temptation" line in the Lord's Prayer. The Spanish version is more "do not let us fall into temptation".

So this Spanish priest may well fell that the variations in the German mass are "corrupt and depraved" because they do not as precisely match the Latin as the Spanish ones, and as some German variants, do.

It is also possible that the Spanish priest is constrained to use a particular form of the German Mass. The Spanish Bishops Conference could have decided on an approved German text for use at German-speaking services in Spain. If they have not then the priest could use any version permitted in Germany or Austria etc.

In 2017 Pope Benedict authorised a review of Liturgum Authenticam, and there are suggestions the pendulum may be swinging back towards encouraging national adaptations.